Forrest, I was thinking more about the other albums I have mentioned, especially Cyborg (EMS synth, tapes, organs, and head full of ideas). Mentioned Timewind was recorded using much less equipment. Some parts directly recorded on 2-tracks.
On X he used his "Big Moog" modular synthesizer plus couple of other synths. He sold it later to Peter Namlook, and Namlook put it on Ebay after DSOTM series (I saw that auction... ). And yes, real Mellotron for choirs.
I'm not familiar with Cyborg. I remember that Timewind was very string synthy. Especially liked side one (Bayreuth Return), though the white noise ending to that nearly jolted me out of my seat the first time I heard it.
Re Klaus' use of mellotron: You're right--his website references a mellotron as of '76. I guess I was fooled by Klaus' reference to a "voice computer" in the liner notes to his Mirage album. A machine with multiple tape loops is no computer! I thought for some reason that he might have been using a Chamberlin, but I guess not.
Currently listening to Hector Zazou's swansong, an ambient collaboration with Indian musicians called "In The House of Mirrors." Very nice and meditative.