used Integrated tube amp under $1500 for Klipsch Cornwall III's?

Pretty much anything will have enough power for the Cornwalls.  At  102db/w sensitivity, even a single ended triode will push them to close to 110db on peaks.  For the OP, looking at the features and sound characteristics of the amp are far more important than power output.  

I would place it on the piano.  You run the risk of the tubes picking up the vibrations from the speakers and feeding back when on the speaker (microphony).  Some amps are more susceptible to this than others.