I compared the mix on the Quadio disc to two different demodulations of the CD-4 LP and one transfer of the Q8, and compared to those (and verified by using both a couple of tools in my DAW including Voxengo SPAN and my own two ears) the quad mix on the Quadio disc needs the front right channel increased by 2.4dB to balance the front pair. You can also achieve the same effect by lowering the front left by 2.4dB but to me it felt like the rears were too loud as a result.
I spoke to Darrell Johnson (who did the CD-4 cutting of this album at the JVC Cutting Center in Hollywood) a few years back and while I don't recall what he said about this album specifically, he told me that either the engineer or producer of the album he was cutting would attend the session to approve the mastering make sure it sounded right, so I have to believe that the 2.4dB boost is what either Gene Paul (who did the quad mix) or Arif Mardin (who produced the album) wanted.
The CD-4, while obviously being sonically inferior to a master tape transfer from a fidelity standpoint has a more pleasing tonality to me, thanks to a cut in the lower midrange and a boost in the upper midrange compared to the Quadio disc. A wide cut centering around 1.5kHz (so it's starting around 600Hz and ending around 3kHz) of about 3dB will tame some of the midrange, and then a slightly narrower boost of about 3dB centering around 6kHz (so it starts around 4kHz and ends around 11kHz) will add some sparkle and presence to the vocals, lead instruments and hi-hats and cymbals.
It also seems like the quad mix has the bass rolled off (this is in all versions, CD-4, Q8 and the Quadio) compared to the stereo mix, maybe with a view toward making it compatible with CD-4 cutting, compared to every stereo version (which was about 6 different CDs, the 192/24 HDTracks and Quadio flat transfers, and one LP) that I auditioned. Listen to the intro of 'Got the Love' to spot the difference - the bass drum 'thumps' in the stereo mix, and 'pops' in the quad mix. The amount of roll-off varies from track to track but you can safely add a low shelf (below 200Hz) of at least 3dB (and up to 7dB on some tracks) to give the quad mix more of the visceral low end impact of the original stereo mix, which I think it needs.