Honestly, i don't think a new receiver will improve your sound quality much if at all. Your speakers are so close to you that they probably don't even use half of the amplification power of the one you have right now.
The only real benefit I could see from a new receiver might be a different brand of auto EQ / room correction. You're probably better off spending on a calibrated measurement mic to see what kind of in-room response you're getting / if you need absorpsion or diffisuion / or if you need to smooth out the bass response with EQ or a second sub.