What is troubling is that when he finished is 'integral' he said ha needed a lot of time to records the Art of Fugue. It took him about 10 years. Mmmm... Ten years for THAT?
Grenzig (who I know personally) is an extremely inventive person. He calls himself a traitor of a German (I don't think I am being indiscreet, he said as much in an Organ conference) and, contrary to the German creed, he likes very low pressure. Now that allows for an extremely flexible wind (your fingering *does* indeed influence the sound) but the basses are not that strong. And, more, you cannot draw a lot of stops together.
But Bach demands strong wind: he himself said that. Let's say at leat 80 mm. The Iberian organs Grenzig is so fond of often have 40 mm... When I play an Iberian organ, I must really strike the keys; and more, I must use at least principal 8' and octave 4' to make polyphony understandable. Now Isoir is very found of flute 8' alone registrations. Of course that does not work.
If one searches spirituality, as Pe-Zulu does, and I think quite rightly, one may be moved either by Walcha or by Christensen. In the harpsichord, the version I favour the most is Gilbert's (even if it is the version from the 1740'ies, and not the last one). Moroney's is a fine, academic one. Leonhardt's a powerful version, perhaps too human, but nevertheless a pure marvel (bad recording, though and also no last fugue).
But the Art of Fugue is beautifull in every way. Even Munchinger (Decca, I think) gave us a sublime interpretation. Using [almost] only strings... A marvel.
The Art of Fugue penetrated me to my very soul. I dreamt about it, played some of it and I hope that Fragment X (that's Christoph Wolff's code name for the missing part of the last fugue) will be found during my lifetime, although I really think it is lost forever.