IMHO the weight of the vinyl has pretty much zero to do with the sound. I have some great sounding albums from the 70's that flex like crazy. The sound is going to be affected by a host of other things, one of which is who mastered the music in the first place. If it was a lousy recording to start with, having it on heavier vinyl or even remastered, won't fix it. Specifically, I am thinking the first Chicago album (CTA), and Carol King's Tapestry - two of my fav albums (you can tell I am an old fart) but IMHO kinda muddy sound, even the remastered versions. And then released around the same time, BS&T 2 - incredible sound and music. My best sounding albums have one of the following guys (all guys) as the mastering engineer - they are normally listed in the fine print on the back of the album or in the liner notes somewhere: - Bernie Grundman; Stan Ricker; Kevin Gray; Steve Hoffman; Roy Halee; Bill Schnee; Doug Sax; George Marino; Todd Wunderlich (MoFi). I am sure there are others, but if I see one of these names, then I know it will be great sounding record. Also the label - Analogue Productions - IMHO - seems to do the best job of re-issues and re-masters. MoFi is also good, notwithstanding their lawsuit and marketing. There are other labels that are defunct that produced some great sounding music - you can find them on Discogs - Umbrella, from Canada; Crystal Clear Records, M&K Real-time; Recut Records (Sweden) - again, a short list from my old memory.
YMMV of course.