The 5.3s are very revealing in exposing the characteristics of upstream components. I agree with many of the above posts that the preamp is probably more critical at this point, especially since the 5.3s present a relatively benign load to amps.
I also agree that you should take some stuff home for a demo, and the decision will likely become an easy one. I'd highly recommend trying the Adcom GFA-750 preamp as it's very transparent and also has a home theater pass through feature that will let you seamlessly integrate it with your Yamaha, which can continue to do the surround processing for movies and surround music. You can then add a stereo amp later for the L/R fronts, which will leave you with high-end components throughout the critical stereo path while your Yamaha powers the center/rears and does surround processing.
As far as amps go(if you go that route instead), I'd consider Sim Audio, which Soliloquy has used to voice their speakers. I'm using a McCormack DNA-0.5 Rev. A that is in your price range(used of course) and works exremely well with the 5.3s--highly recommended if you can find one. Since you're obviously using your system for HT as well as stereo I won't bother with tubes.
As a fellow owner of 5.3s I can tell you that proper placement will be as or more important than your components in getting the most out of these speakers and your system. In my experience they don't really open up in terms of the soundstage and midrange unless you get them around 4 ft.(or more) from the back wall. This may not be possible in your situation, but the point is don't neglect speaker placement in your quest for better sound. Best of luck.
Tim
