For the prelude, I tried to evoke some inexorable figure,something like a giant which doesn't walk that fast but that nothing is able to stop.
The fugue is treated as an "old fashioned" terraced crescendo, to avoid 15 minutes of continuous loud plenum.
There are questions and controversies about the fugue which would have been composed much earlier than the prelude. After practicing it, I feel it as fitting the prelude like a glove (my opinion was quite different before practicing it and I then didn't like this fugue).
The registrations use generously Regensburg's resources for providing "Gravitat", perhaps too much...