Well, I gave it another shot, and I will admit I was much happier on my second listen - particularly with the volume cranked substantially. Still not 100% satisfied with the "WGO" songs, but some of the other Seventies tracks ("Let's Get It On" and "Trouble Man" in particular) were pretty stellar. I'll admit, my initial listen wasn't under the most ideal conditions (it was sort of playing as background music while I was doing other stuff in my apartment), but I guess that's one of my general complaints for surround music in general - you more or less have to plant yourself in one place for the entire thing, or else you're bound to lose part of the mix if you move around a lot!
At any rate, I'm sure I'll be one of the first in line to pick up the "Let's Get It On" SACD (and anything else Motown decides it wants to give us, especially any more Marvin). As always I just hope they are able to maintain the same "groove" as the originals.