Building speakers are normaly a long and dificult process. It’s not just selecting the right drivers and putting these into a well designed enclosure. The real challange is the crossover.
The crossover will have a high impact on the resulting sound, and requires a lot of knowledge, possibility to make good measurements, possibility to make simulations and a good deal of patience.
Using a digital crossover can make the process much shorter and the chance of a succesful result much higher.
Using a line level crossover, either digital or analog, of course requires more amplifiers, one for each driver, but in general these then don’t need to be as big as otherwise.
But making you own digital filter board is of course not easy at all, and making a suitable PC program that can be used for ajusting the thing, is even harder.
I just came across this small board made by miniDSP.com which is sold for only 99 USD. It has two analog inputs and four analog outputs, enough for either a four way active filter or stere 2 way.
The PC software used for ajusting the filter, comes for 10 USD. Given the time and cost of development it looks quite a good deal.
The board is based on the Analog Device ADAU1701 chip, which is the hart of the design.
Only things that are missing, I think, is ability to play from a digital source, through either SPDIF or USB, or both
But the SPDIF interface should be something you could add quite easily, as the ADAU1701 has 4 I2S inputs …
Would love to try this out with a small two way speaker and 4 amps …. -P
Have any of you tried out these small wonders????

I have seen it too.To me it looked as it was possibly to use it to EQ as well.
I’m going to buy one myself. There is a spdif digital in/out module (65$) as well and a full digital amplifier (i2s input)on it’s way
I have bought the whole lineup. You can see it here.
http://www.diyaudio.com/forums/digital-line-level/164742-my-minidsp-setup.html