Upon cursory review, I think this thread explains the pre-echo phenomenon the best. Here's a similar explanation from an outside source:
"When you master an LP, you have to put each groove as close to the other as possible. As a result, in loud passages you sometimes can hear bleed-through from the previous groove. (A similar thing can happen in the master tape when one layer of tape magnetizes the next layer on top of it in the reel.) Compromises in the sound must be made to press music in this format, and bass is what mastering engineers have to compress and attenuate the most."
Quoted from:
The Joy of 45 Collecting: Why 45s Sound Better Than LPs (classic45s.com)
I'm hearing this in almost every quiet passage of music on LP's, throughout the LP, not just one revolution before the start of the LP. This effect must induce some low level distortion, possibly 1-2%.