Don Firth's ideas on the history of equal temperament were the orthodoxy fifty years ago and have long since been abandoned by experts. Nobody in the early music world now believes Bach made significant use of it. There is also pretty good agreement that we don't know what he *did* have in mind for the Well-Tempered Clavier - could have been one of of Werckmeister's tunings, could have been something slightly different. It was certainly *not* routine in Beethoven's time (he was one of the best writers ever for the natural horn, which can't get anywhere near it). Some of the flute fingering charts on my CD-ROM are even later and still have differently pitched fingerings for A flat and G sharp.

Whether you can hear different tunings depends on the kind of music. Recorder consort music is at the extreme - very pure tones played rather slowly, so beating is utterly obvious. Barbershop quartet singing is not far behind. The sound is immediately identifiable to just about anybody.

Cajun accordions are usually tuned in the factory to meantone, and Scottish clarsach and mediaeval harp players often tune to Pythagorean intonation if they're playing solo. Empirical measurement shows that whatever classical string quartet players *think* they're doing, their actual intonation scheme is mostly Pythagorean.

Pianos certainly can be tuned in non-equal temperaments, and for historically informed performance (e.g. of things like Haydn sonatas) always will be.

Someone who knows what they're doing can retune a harpsichord in meantone with astonishing speed and no more equipment than one tuning fork. The tuning procedures described in 18th century books really do work.