I'm definitely no expert in tuning, and I confess to not even really having a good understanding of the various systems.
Our organ, as it has been explained to me, is NOT tuned in equal temperament, but it is substantially close. Our tuning was compared to Valotti, although it is not exactly Valotti, and has been also compared to the tuning that John Brombaugh pioneered. http://plaza.ufl.edu/wnb/baroque_temperament.htm
The voice is very adaptable to various pitches, and tuning, say, to A=432 or A=442, or even A=418 ("Baroque" pitch, a semitone below A=440) generally presents no problem to singers ... except the few that have absolute ("perfect") pitch (who can be a problem themselves if their absolute pitch is equally tempered).
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