Which speakers do you have ? Depends a bit on how big & bass-heavy the speakers are, front vs rear port etc...

Probably a foot or so away would be sufficient, but you might want to pull out a bit further so the speaker fronts are not behind the front of the piano.

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8