I'm friends with a physicist who for years was lead vocalist in a band. He now works as a professional audio consultant, mainly for prosound applications (his clientele is spread across four continents). He is very good friends with a concert pianist. The pianist has come to respect the physicist's ears so much that he won't release a recording until his physicist friend has heard and evaluated it.
I'm also friends with an aerospace engineer and former concert musician whose runs a rather successful loudspeaker company, mentioned several times in the latest edition of The Absolute Sound as either best or among the best speakers at CES (and he doesn't advertise so there's no back-scratching going on).
Both of these individuals design loudspeakers with physics and psychoacoustics as their primary tools. Neither of them design "by ear", and for listening evaluations both use other people rather than themselves.
In my opinion the key is knowing what sets of measurements, along with their proper interpretation, will correlate with human hearing perception. A musician may well know how to recognize when it sounds right, but when it doesn't (and the first try never does) how does he accurately identify and resolve the problem? The most interesting loudspeakers in my opinion are consistently those employing intelligent acoustic and psychoacoustic solutions, and that's the province of science rather than art.
