I’ve a few Quad LPs, Santana Caravanseri on the CBS label, Pink Floyd’s Atom Heart Mother and Dark Side Of The Moon plus Deep Purple Machine Head and a few others, including some classical. As you say they sound that bit different, enough to collect them, which I’m doing when I see them. The 2 Floyd ones in particular are well worth seeking out.

there were SQ (columbia) matrix, and CD-4. The CD -4 required a special stylus but the sound was discrete, not matrixed. and Sansui had QS.

I picked up a Quad version of Super Session by Bloomfield, Kooper & Stills last year. Does sound like a different mix compared to the Stereo release

I had 5.1 setup for quite a few years. Got into a lot of live music on DVD and Blu-ray. I preferred live music to studio. Now I have gone back to full analogue stereo. A good stereo setup can sound 3D..or is it 4D... .

Columbia was already in use in the UK by EMI. All the early Cliff Richard albums were on Columbia.

I have John Lennon's Imagine in Quadraphonic. It is probably the opposite to your Simon & Garfunkel album. John's voice is really far back in the mix on the Quad version, almost sounds like he's in another room. Interesting to own but not my go to listening experience for the LP.