Which Receiver for Sonus Cremona Auditors

If price isn't a big concern, get seperates. You will get better quality (usually) and can concentrate on a lower powered, higher quality amp. I'd suggest Anthem, Parasound, Sunfire or similar. Spend the money on a newer pre-amp with all of the features you need and maybe get a 5 year old amp with 100wpc since most of the new ones are up around 200 watts.
DO NOT PUT SPEAKERS ON THE PIANO-other audiogoners may/may not agree with me. However, not only will you get strange resonance from the piano due to vibrations, you may end up knocking the piano slightly out of tune??? I wouldn't do this to my piano.
Finally, the sub: Even though you don't want to bother with loud sounds, you really need it to round out a 5.1 soundfield. Many good pre-amps/receivers can send the bass to the right and left front speakers. However, this can be a strain on the amp driving them as well as on the speakers to produce that much bass. Try the system without a sub at first and see....
You may want to try the system without rears also for now... If you have great fronts (which you do) and really nice pre-amp and amp, you just may be able to skip the rears and sub for now. It won't hurt anything to try.

You need the best amplification you can afford in order to take full advantage of the speakers. If I had to go multichannel, I would choose a RMB-1077 Amplifier with either a Rotel pre/pro or an Audio Refinement processor (~ within your budget).
I second the idea "no speakers on the piano."
SF also makes "wall" speakers, you might want to consider those.
Good luck.

Arcam.