Bob, thanks for chiming in on the tubes vs. transistors point.
While the last round of tube gear represented a mature technology and the first waves of transistor stuff were a little primitive by comparison, I don't think this really accounts for the lesser quality recordings that I was referring to. For sure, there were lots of bad sounding 60's and 70's records made on tube gear, and lots of great ones recorded with solid state.
I strongly suspect that the lesser quality recordings had much more to do with gear that needed repair or was being used incorrectly, gear patched in ways that it wasn't really meant to be patched, ground noise, mods and homebrew gear that didn't quite cut it, record levels set way too high or low, machines with their bias set for the wrong kind of tape or not set at all, wacky EQ settings, sloppy mic technique, etc. etc. etc. In other words, human error.
As stated before, I think the industry grew up a whole lot during the 60's and 70's -- guesswork was replaced with more educated engineering, and better monitoring revealed problems that may not have otherwise been apparent. Also, maybe better mastering systems and the advent of "hi-fi" forced engineers and studios to deliver better sounding product.
Of course, the above includes a lot of sweeping generalizations, and plenty of exceptions could be cited. (To me, one glaring exception is Stevie Wonder's 70's albums -- Innervisions sounded fabulous; the next few didn't even come close. Come to think of it, "Contract On Love" sounded pretty good too.
Who knows...)