anyone familiar with Sonys CDX-C90/4000

The crossover section is only a small part of an amplifier's sound.  Two amps will always sound slightly different.  I haven't heard either in a controlled environment (same speakers, etc).  I think the D7 series by Diamond is probably in a class above the JL. 

How do you like your xtants? I have heard good things about those also.

I did a install that consisted of a old esoteric 7152 (now diamond d7's) and it sounded awsome on a set of JL comps xr comps. He had a OK source unit (sony xplod junk) vs my C90..... I had the JL 300/2 on a set of polyk focals in my own car  (with the TN51 tweeters) and it didn't sound like anything special. He had a van halen live CD that sounded like a damn concert in his vehicle, put the same cd in my car and it didn't sound as musical.

I have had experience with both... I installed a esoteric 7152 (150x2 at 4ohms) on a set of JL 6.5 comp xr's with a sh*tty xplod deck in a friends vehicle and it sounded like a damn concert hall in his truck. I had a 300/2 jl (150x2 at 1.5-4ohm) on a set of focal polykevs with utopia tn51 tweets and my ultra clean C90 deck and it didn't sound as musical. Maybe I am crazy but i think the esoteric on the JL's sounded better than my setup.

hah, i wrote that post out twice thinking the first didn't post.

The 4000X processor is an absolutely killer processor.  The only issue most people have with it is the crossover frequencies available.  They are very limited.  Check out http://www.matronics.com/xdp4000x-list to download Sony's DSC software used to control the 4000X to see all it's funcionality.

As for the amplifiers, I don't have any experience with either.  Good luck though!

Steven Kephart

Adire Audio