I suppose this is the best place to post this rather than an ongoing thread in the Coffee House.
In Dave Cousins's autobiography he briefly describes the technique used to create the guitar tone on "Down by the Sea" on the "Bursting at the Seams" album. I have always been amazed by that tone.
The guitar amp was placed under a grand piano and the pianos sustain pedal held down with a heavy weight so the piano strings resonated. The microphone arrangement is not mentioned. Listening to the track again now it makes sense.