I can only humbly verify what has been previously stated - and so expansively & succinctly.
After purchasing Karl Richter's exquisitely fine (and brilliantly re-mastered) 'Bach Organ works' (3 x CDs, from 5 x LP's) on the DG label, I just HAD to listen to this man more: such virtuosity on the organ!!
(I had, at one time, a cassette-tape of these performances, but I wanted a modern 'remastered-onto-CD' version. This is not currently available on Teldec, so please, re-issue it.)
First: the 1959 AAD sound is quite brilliant and full for its age. I detected (through my Sennheiser headphones), no real hiss or brittleness - which is so characteristic of early sound recordings (and this one must be VERY early stereo.) If anything, the treble could be sharpened-up a little more.
Richter appears to have used a much larger organ than the Chamber Organ Handel would have specified, so Richter's organ part IS powerful - but, in these 2 master composer & performer's hands, it is a true, sonic treat.
I only have acquired 'Volume 1' so far - but it's a start, and I will keep e-searching for those other two elusive volumes.