I always wondered if the Quad mix was limited by the multitracks having been recorded in less than ideal conditions in Nigeria. After watching a BBC docu on the making of BOTR, I learned they recorded 8-track in Nigeria, and upon return to London, dubbed those 8 onto 16-track for further overdubs and sweetening.
As much as I love this album, I think the Quad mix could have been a little more adventurous, especially considering the lavish instrumentation. I detest what I call "Quad Black Holes" which are silent channels in what should be a surround experience. I find that BOTR is silent in the rears for too long on some tracks. Or, worse yet, the same thing playing in the back is also heard in the front. This title does like to use the phantom left and right channels far too much.
Still, I give it a solid 9. Great music, decent Quad mix, but I feel since it's a "feel good" type of album, they missed out on some four-channel playfulness.