I can't imagine the expression in Bach's face, standing in front of a great "monster" as many organs in the world today, and I agree with contratrombone64 saying that, probably, could play 24 hours without stop!?!
Dear Tom, I also am studying the Passacaglia, and, like you do, the problem for me is still the registration!?! I heard a friend playing it with mixtures in all manuals and 16' and 8' reeds on the pedal from the beginning to the end...:I'm sorry for people who likes to play the Passacaglia in this way, but... IT'S a LITTLE BORING!?!!! It's true: the continous crescendo you can see reading the pages of this piece of Bach, couldn't need, may be, of changes: infact, Bach at his time, had two or three manuals orgnas, in which he could make two or three different "sound levels" even with mixture in both of them. The diapason of each manual's principals and mixture was very different from the others, giving different levels of "pleno". And that's the first possibility you have!
Bach also loved so much the "gravitat" of the 32 foot.. a rare stop at his time!!!
But try.. to think at Passacaglia as a continous SOLO theme..you hear fist in pedal and then bouncing from a voice to another...
If I could have a 32' stop in my organ, I surely would begin the pedal theme with it, and with a deep 16'...and on the manual, a Principal 8'.. giving precision (but always soft than pedal theme playing) to the chords!!!
Manuel