I don't agree that this release is 'muffled' or from a bad tape copy at all. The quad mix is perfectly balanced (levels-wise) and if it's not a flat transfer, it's close enough to it.
The front speakers are generally bass guitar, fender rhodes/clavinet, and drums. Mike Clark, The Headhunters drummer, didn't use a lot of cymbals nor open hi-hat. Which of those instruments are you expecting to have a lot of treble response?
When things that do occupy the higher end of the frequency range (analog synthesizers, flute, saxophone the various bells during the percussion breakdown on the first track) come in, they don't sound muffled at all and have plenty of that hi-rez 'magic' that never fail to make the hairs on the back of my neck stand up. Sometimes I think people have been so conditioned by modern remastering where the high end has been EQ'ed like a ski ramp and the hi-hats are sizzling like bacon in a pan that anything less sounds muffled to them.
You have to remember these albums were originally recorded with the intention of mixing in stereo, where all these instruments had to sit on top of each other in the mix. They couldn't all occupy the same part (treble area) of the frequency spectrum, each had to have it's own 'area' carved out. I recall similar complaints/observations about the Loggins & Messina quad mix, but that's what happens when you spread the instruments that occupy the upper end of the frequency spectrum (guitars, horns, hi-hats/cymbals in L&M's case, and flute/sax/analog synth, etc. in the case of this album) and put them in the rear speakers.
I've owned the original stereo vinyl of this album, the late 90's CD, and the 96/24 digital download, and I've heard the SQ LP and owned the Q8 and the tonality of this release is very faithful to the sound of the original. If you like the other Headhunters quad SACD from the early 00's, I don't see how there's any way you could be unhappy with this one, it's fantastic - maybe it just needs to be cranked up to reference volume to be fully appreciated.