'Box' speaker that reproduces the piano well.

This is the Internet.

Agree with ATC, but your 35 wpc amp would probably not have enough juice to drive them properly so you'd likely be limited to using your ss amp.  Best of luck. 

Zu Druid Mk IV with Densen DM-10 MkII, First Watt F5, Leben CS-300. 

Genesis Vs do a wonderful job with piano, are well under your budget, and I suspect would work with either amp.  For a different presentation, the Audio Physic Avanti IIIs are nice as well.  You can also occasionally find Verity Parsifal Encore's in your price range, which I would also highly recommend if you are interested in an accurate rendition of the piano.  However, for the latter two speakers, I am not sure how they would work with your 35 w/ch tube amp -it really depends on the amp.  For example, the Vac Renaissance 35's should work very well with either (depending on room volume and other system factors), but other low powered tube amps may not be the best choice.

Avanti1960 has a good point. Smaller speakers or monitors will improve the sound. Piano's to me sound boxy. Smaller speakers don't have all the cabinet resonance of large speakers so they are cleaner sounding.