Best Of Volume II and Road Food have nice wide, aggressive mixes. I would love to see those come down the pipe. #10 is almost one of these mixes, except they decided to put a different guitar in FL and BL, and then Burton's keyboard in FR and BR. Makes for a lopsided mix. But the rest is your typical lead vocals/bass/drums in the front, backing vocals in the rear. So close....
Since on the topic of RCA Quad, any of the Elvis titles would be great, but they'd probably have to pay through the teeth for licensing. The Jerry Reed titles all have great mixes. Waylon Jennings Lonesome/Onry/Mean is very good. Maybe even the John Denver? Would be interesting to see if they can dig up the unreleased John Denver Quad. I suspect it was "Rocky Mountain High", but it was only mentioned as an "upcoming" John Denver title in a 1975 Billboard magazine. (The same ad mentioned a David Bowie Q8 that also never came around).
Back to the topic of the Guess Who though, I would love to hear the entire Live at the Paramount album in Quad. One song was remixed for Best of #2, chances are high that they mixed the whole album. Or better yet, "So Long, Bannatyne". I can only imagine "Pain Train" in Quad with all those vocals tracks. They were on 16-track equipment by then and surely would have used it to their advantage. *sigh*