And least we forget, Parsons also didn't do the original DSotM stereo mix - that was done by Chris Thomas. In fact, if I recall correctly, I think the group was actually unhappy with Parson's original mix and handed the job to Thomas since he'd had nothing to do with the original recordings, and therefore could be a bit more impartial in handling the mix. (That's actually not an uncommon practice these days.)
That mentality might actually explain why Guthrie was chosen to do the surround mix instead of Parsons, as well. For all its perceived faults, I think Guthrie's mix tends to preserve the "feel" of the original stereo mix much more than Parson's Quad mix - and the stereo mix was the one that the band labored over much more intensively back when the album first came out.