I've settled on senn hd-650 and and akg k701 for headphones. Are they good enough? ( I thought I'd be able to choose one over the other, but alas both seem to serve different purposes, depending on my mood, and both seem pretty good). I still prefer speakers, and since most of the music I listen to is designed for playback on two speakers, that is what I generally use.
I could not afford your room - well, I suppose I could, but it would take up more space than I want to give up, serve more people than I need, and just
would be impractical for me. I assume your home listening room is not a duplicate of your testing room - but I could be wrong . . .
Musicians listen for different things than the audience. And an instrument sounds different while being played than it does while being listened to. So it does not surprise me that musicians are no better at listening than "normal" people. It does surprise me that people who listen to a lot of music are not better at your tests/training, but I'll take your word for it.
Do you pay your testers? If so maybe I could apply and take a road trip. . .
Of course, I maybe useless - we won't know till you release your training software for general use