I'm using the Alpine PXA-H701 processor myself. Marvelous little machine I find. For a stereo setup, you get a 4-way active setup. Front 1, Front 2, Rear and Sub / Center. (center can be changed to left sub if no center is used). The sub x-over can't lowpass any higher than 200hz. The rest of the outputs, you can set your x-over to whatever frequency you see fit and the curve to 6db, 12db, 18db, 24db, or 30db / octave. And, this can be set independantly for the left and right channels. You can also set attenuation for each channel independantly from 0 to -12db (-13db shuts it right off). Then, wether you use the graphic EQ or Parametric EQ is up to you. Graphic gives you 31bands of all speakers except the subs. Parametric gives you 7 bands for each channel and 2 for the subs. And of course, time alignement for all 8 outputs.

I've had it since x-mas and I'm still tuning ! :P

 

I have a DQXS and a Kenwood DNX-8120, with the pair there is nothing you can't do

 

By now you have noticed that the predominant opinion is the big processor and lots of extra amplifiers and wires will give you an incredible amount of control over the system. Will this improve the SQ... maybe/ probably. It is an added level of complexity that some are and some are not prepared for. If you do your homework it is great if you dont ... heck if you do your homework a non active system can be quite satisfying too.

The advanced Pioneer, Clarion, or Eclipse HU will give you a good % of the control of the Alpine 700 series or the Audio Control for a fraction of the price. Continue with your search, if you think you can do this without getting trapped in a spiders web of RCA's and speaker wire then it can be an incredible step forward.

Also Zapco (among others) is putting processor's in their amps so an active setup can be a little simpler too.

 

I did it quite simply, andf only used 2 channels of input to the amp.

I have Dynaudio MK242 components, Kenwood DNX 8120, and an Image Dynamics Q 450.4 amplifier. The amp has a bandpass crossover on channels 1-2 which I used to power the mids, then the tweeters are powered with the remaining 2 channels and adjusted via the on board DSP in the Kenwood.(which doesnt take much even for being on axis to me, still sound great.) I time correct for the mids and adjust the tweeter volume accordingly.
This is my first time running active setup in my own car and it takes some more screwing around to get it right but when it is right it is the way to go.