I am not a technical judge of voice because different people singing the same song all sound different. Pavarotti, Domingo, and Lanza singing Nessum Dorma don't sound alike at all.

The "voice" of one Steinway Concert D is pretty much the same as another. The same person playing the same piece on different Ds will probably sound pretty much the same, but different performers playing the same piece on the same D will sound different because of differences in interpretation, style, tempo, phrasing, etc., but they can each be evaluated according to the same standard of observing the dynamics.

OK, I stick with piano and you stick with voice. Fair enough?