I think Queefee makes some good points. I think your most noticeable overall improvement per dollar spent would come from implementing Audyssey into your system in some way if you haven't already (I'm not familiar with your processor). After that I'd try to match the front speakers, and then maybe a second sub. By the way, this is from a home theater perspective. If this was also geared toward improving 2-channel that's a whole different discussion. Best of luck.
Improve my system
I got the mod bug recently but don't know where to make my next upgrade. Right now i'm running a Yamaha DSP-A1 along with the BK ref 200.5 amp. My speakers are Kef Q70's for front surrounds, Energy APS5-2 for mains, Kef 200c for center, Kef Q70's for rears. Energy microstar 12.1 for subwoofer. I'm using a sony PS3 for my bluray player. I got about 2k to spend on a new toy. What should i spend it on?