Advice for SQ system

Greetings everyone. I am a new member and have enjoyed reading the posts on this site for the last couple of weeks. I have been installing stereo equipment in my cars since I was 16, (I am 32 now). and every system has topped the one before. As I listen to much more music in my car then at home, I want the ultimate SQ system for my car. Most of my system so far is old school Sony ES. CDX-C90 HU, XDP-210, 2 CDX-91 changers, XM-2100G amp. For speakers, DEI 2065 component set for the front, and Infinity Kappa 6X8 in the rear. Those will be ran by a Sony XM-754HX. I will be running fiber from the HU and both changers into the XDP-210. I am still looking for some good quality subs that wont break me, and that I can run in a relitivly small enclosure while keeping good SQ and frequency response. This is going into a 98 Mustang wich has a decent size trunk, but I would like to keep some trunk space available. I hope I didnt ramble too long, thanks for any advice you can give.

I should also mention that the subs will be hooked to the 2100G which is 100 X 2 at 4 ohms and it is 1 ohm stable. 240 X 2 at 1 ohm.