The differences in sound between amplifiers is inconsequential compared to the difference that the speakers make, unless one of the amps is really bad. You want a better sound, you will have to shell out money for some very good speakers. The Infinity Alphas are a good speaker, and it will take some money to significantly top those. I would be looking at some of the middle lines from Revel or RBH, Philharmonic, Ascend Sierra 2s, or Kef Reference. If looks are not a big deal, also look at some JBL LSR6332 monitors or Emotiva Stealth 8s. I would be looking at a couple good subs too, and proper subwoofer placement. Look into a pair of Hsu ULS-15 mk2s and also something that can measure and EQ the subs.
What is more is your room sounds like it could be very acoustically lively with all those hard surfaces, and if there is no way to tame the acoustics in there, you might as well just keep what you have, because the room might put a limit on how good the sound can get in there.