Which Receiver for Sonus Cremona Auditors

Arcam.

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.

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.