AF released several quad SACDs that still get regular rotation here, but for me Spectrum is the best of the lot, both in sound quality and content, as well as for the fact that it was a previously unreleased quad mix. Many of the AF quads suffer from a rolled-off top end as a result of their decision not to master the quad mix - The Guess Who's Best Of, Vol. 1 is probably the worst offender, but even Herbie Hancock's Thrust and Loggins & Messina's self-titled album need 3-5dB of top end to match the tonality of the stereo masterings on the same disc - but Spectrum suffers no such problem. I spent an age trying to fix the 5.1 DVD-A mix of the album, but the quad mix knocks it into a cocked hat so spectacularly I don't think I've even given it one listen since the SACD was released.
As far as chart positions go, I think the more time passes the less relevant they are to anything, especially a half-century after the fact. Sure, "good" and "commercially successful" are two largely overlapping circles, but there's lots of good music that didn't do that well on the pop charts, and even more music that charted that hasn't stood the test of time. I'm not bagging on Bread (especially as the number one proponent of the David Gates First quad mix) or the Doors but if the same logic was applied 50 years from now and a label was reissuing surround mixes of today's music, it would be saying that Taylor Swift and Billie Eilish were the "best" candidates and not Steven Wilson and Porcupine Tree or something like that.
My only (small) misgiving with the quad mix of Spectrum is that the Jan Hammer synth solo that opens Quadrant 4, which slowly pans from left to right in the stereo mix, is locked into the left rear speaker in the quad mix for the duration. It seems like an obvious candidate for a 360-degree around-the-room pan and I'm not sure why they just left it in a static position when normally engineers take relatively great pains to mimic the nuances of the stereo mix, or often exceed them when it comes to whizz-bang ping-pong effects.