I wonder if drones might provide a different type of "ear/mind training"? Matching external sources? Useful for out-of-tune pianos, horn, and other string players. I remember a gig once in which the singer was so off-pitch, I stared at my left-hand for a good minute or so trying to figure out how I could be SO far off-pitch. Turns it, for once, it wasn't me!
Been there-it's torture, and can really make you doubt your ability to play in tune. A couple stands of bad 'cellists can have the same effect.