I dont think the HD58X sound as good as the 660 S does, having had both, The 58x sound closer to a 650 to me, and not quite as good as that either. They sound great for 150 bucks, mine didnt have the issues that OP described, they just sounded quite dull (same problem I have with 650). I had one of the under 500 serial numbers as well.
The 660 S is the best of the 6xx line IMO, and the best sounding reference headphone in general for music (movies and games I would give it to the Dt1990). It is worth the extra money, even with EQ I never got either the 58x or 650 to sound as good.
Look forward to seeing solderdudes measurements, maybe there are some defective ones out there, might explain why there are a few really bad reviews in the middle of mostly positive ones.