1. Invention
You are going to invent your very own loudspeaker system. "Your very own" means that you may try to realize a soundreproduction which gives the "being there" illusion, with a wide, high and deep stereo image, with extended bass and high frequency reproduction, a system which will give speed and magnitude as if you were in the concert hall. That is the extravert way.
Or you just may want to build a very small two-way system which does not have the impact of large panels and big units, but lets you imagine a perfect world of harmony and inner detail. That is the more introvert approach.
There are many possibilities in between. There are many ways that lead to happiness.
You can look for a neat and precise reproduction or you can achieve the live experience.
I myself want to have the experience of the live performance sound in my listening room. A grand piano should just stand there in full bloom. A violin should have that same live character as when my brother is playing a Tartini sonata or a Kreisler piece. A drum should show its vibrating skin. Already for that instrument you need a large surface to reproduce the lows and the lower mids. And the brass section should have the snoaring, high, top frequencies which are so characteristic of that warm and yet scintillating and chiseled sound. Yes, writing about it is easier than making it happen.
The first thing to do is to learn what the sound of acoustic instruments is like in reality. Even if you are not too fond of classical music you should go to a concert hall from time to time to listen to a symphony orchestra or a piano recital. You should go to a jazz club. You should pay attention when a fanfare plays in the swuare of your hometown or just passes by in your street with that firm sound of the drum and the dynamics of the glockenspiel.
While constructing you will learn that sound is not only frequency and decibels. You will learn that it is also amplitude and that the low frequencies produced by your bass speaker should have the necessary amplitude to give the mid and high frequencies a good basis to produce the harmonics in the respective registers they are responsible for.
Amplitude can only be achieved by dynamic loudspeakers and not by electrostatic loudspeaker panel and isostatic speakers.(br)If sufficient amplitude is lacking in the bass or an irregular amplitude curve (like in ported speakers and transmissionlines with a dip around 60 to 75 Hz.), than the mid and high registers will be restrained and will repeat the harmonics of the irregular patern in the bass. But once you have heard a big system like a Klipschorn which -because of the high efficiency- can make the step from a tender pianissimo to a realistic, but well controlled fortissimo in a few milliseconds, untill you have heard a Cabasse Atlantic which reproduces a Bösendorfer Imperial Grand as grand as real life, untill you have heard the impact of a drum accompanied by cymbals in a symphony orchestra, or have heard the live performance of a violin in your listening room, you may aim for big speakers. Only then of course when you have enough space and your wife or partner does not object.

2. Transient and harmony
Although harmony, phase, wide range, transient response and amplitude are all related and depend on each other, one can say that harmony is the first important aim to be realized. You can find an extremely harmonious configuration without being completely correct in all registers. I have heard fantastic active speaker configurations (some of my own) which made the hairs on your arms stand up and could move you to tears just because of the impact the harmonious sound had, yet the system was behind in many other aspects.
Harmony depends on the correct phase of the entire system.
Transient response depends on phase. If the fundamental frequency of an instrument (say 40 Hz.) is not being followed by its harmonics (80, 160, 320, 640 Hz. etc.) with their natural sound levels, the plucking of a base player in a quality jazz recording does not sound real.
Transient response also depends on the bandwidth of the system. That is why in the golden days of analog quality phono stages boasted on a bandwidth of 400.000 Hz. instead of the limited 40 or 50 kHz. The drawback of most of those devices was that they picked up signals form radio stations (you could hear Radio Moscow via your cartridge) and transmitters for pagers.
Bandwidth also depends on the source and on the amplification. Analog sources provide a far better transient than digital formats.

3. Best loading
You are going to reinvent the loudspeaker as it were - if you have not already done that earlier by building a passive speaker system.
Be aware of the fact that an active system is different from a passive system. Since there are no passive filter sections (no coils and capacitors), it is necessary to determine a different and appropriate volume for each and every unit. The interesting thing is that you can determine the loading of each individual unit, not only by calculating with formulas and measuring through programs, but also by listening to the performance in practice. In this way you can create the sound you want. The basis is of course the sort and quality of units which you have chosen.
For the lower mids I have built cabinets of which the volume is adjustable. The surface of the cabinets are squares, but the back is a plyable wood panel. You can adjust the panel in order to find the best back loading of the unit.

4. No hurry
Make sure that you have loudspeaker systems to listen to, even if these are simple but effective 2-way speakers like those of Mission Electronics or Bowers and Wilkins. This means that you will be able to enjoy music while you are going the long way of building an active system. So you need not be in a hurry to finish your active system (if it ever will be). While listening to the smaller speakers, you will be confronted with correct sound and this will make you realize if you took a wrong direction somewhere on the way.

5. Identical
While devising and optimizing the active system you should use (two or three, depending on the configuration of the system) identical power amplifiers. It is not advised to use different makes and types of power amps unless you can measure that the phase characteristics are indentical over the entire frequency band.
Preferably simple, but correct loudspeaker cables should be used in the beginning for your system. Or cables that do not have anomalies and/or manipulated and/or do not ask for very heavy power amplifiers. Use Supra Cable for instance, the simplest Monsters or a sophisticated, well balanced type like Cardas. Be sure to know the qualities of the cables you are using. Simple cables have the advantage that they are not expensive. You will need at least 2.5 m for every channel. Use the same type for all channels. Uniformity is the key word in building a good system. At the end you always can connect the speaker sections using very expensive cables and adjust the system slightly if necessary. The choice of loudspeaker cable depends also on the sort of amplification you choose: tubes, transistor, high power, low power, all in relation to the efficiency of the loudspeaker units.

6. Sources
Be sure to have a good source: CD-player and/or turntable. You should know the behaviour of the source, its qualities and its limitations. Do not compensate for shortcomings of your CD-player and phono section (cartridge, arm, phonostage), but make sure these sources are performing correctly. If you are in doubt of the correct performance of your high end turntable and 4 kilo bucks cartridge, use a good but simple specimen that has justified its existence over the years as a good analog source. Momentary I play with a vintage Shure M91 moving magnet cartridge and am amazed and enjoy a very good overall performance with a very good midband.
Remember that room temperature will specifically alter the performance of a phono cartridge and of tweeters filled with ferro fluid which is more supple in summer and at high room temperatures, than with an everage 21 degrees Celcius when viscosity is lower.
As for the CD-player, just leave it always switched on. I have switched mine on several years ago and it was only switched off a few times when dusting or changing its place.
Of course quality interconnects between source, preamp, electronic crossover and power amplifiers should be used. The affordable Cardas type are an excellent option. Whatever your choice is, make sure that the interconnects from the crossover to the power amps is of one and the same brand and type.

7. Listening room
While constructing and optimizing you will notice that you will compensate for acoustic anomalies (imperfections) of the listening room. Standing waves will easily be generated. Your starting point should be a given position of the speakers which can be adjusted slightly if necessary when you are progressing. Read the article about Loudspeaker Placement to get more ideas and information that you can apply to your listening room. Place the speakers some five to six foot apart, to begin with. And determine a good distance from the side walls and the back wall. Distances depend not only on the room, but also on the width of the baffle and the size of the units and the position of the units.

8. Surround sound
If you want to extend your system to surround sound you can do that later taking your active system as starting point. Since the additional surround speakers are restricted in their frequency bands, they only will influendce the reproduction of the midband. In that case the best placement of the rear and center speakers have to be found without altering the active system (if it is optimized). This means that you still can listen to old, fantastic high resolution analog sound when you wish to. You would not give that up just for some heavy door-slamming and car crushing in an action movie. Would you?

9. Coupling
Stands or racks used for the power amps should be identical. Do not forget that when a rack gets heavier by placing more heavy power amplifiers in it, the sound reproduced by the units can alter slightly.
It is a good thing to couple the bass enclosure to the floor with spikes, right from the beginning.
If your listening room has a concrete floor it is a good thing to make holes of 10x10mm in the carpet so the spikes are completely free standing and not touching the carpet..

10. Configuring
Electronically as well as acoustically there are variables which need to be controled and adjusted. First there are the levels. These are very critical. You will learn how to adjust the levels of the various sections in relation to each other. A slight turning of the volume of one channel can alter the sound significantly. Especially when using 6 dB slopes, the turning of 1 mm of a volume knob can be enough.
You also will find that you need a stepped configuration of the units and that the vertical distance between the units is as important. You have to determine the correct distances. They relate also to the levels and the slopes.
The choices of slopes are determined further by the width of the baffle and vice versa.
The choice of 6 dB or 12 dB slopes determines the ease of the reproduction. Slopes of 6 dB per octave would be the best option. In that case you need to choose units that are very linear and have a natural roll off.
You will find that combinations of 6 and 12 dB will still give the best phase characteristic. A 12 dB low pass for the woofer and for the tweeter a 12 dB highpass, while the mid band has slopes of 6 dB at each end is a very good configuration. Personally I do not like 24 dB/octave slopes because they tend to make the transition from one register to another rather difficult and they will easily restrict amplitude.
As said earlier, levels are crucial. The slightest increase or decrease will effect harmony, transient response (slam) and distortion. Good luck!