When the sound falls apart at higher volumes

[Andrew and Rebeca Anderson]

I recall that this phenomenon has been referred to here.  Have a 
piano in a rather live environment (tile floors, not much on the 
walls).  At higher volume playing the tone seems to "go out of focus" 
and gets rough/harsh.  I've wondered about what causes this and what 
approaches may be used to tame it.

Would adding impedence to the bridge help this?

Andrew Anderson