I just got HE500s last week and the bass impact has suddenly disappeared. I had to twist the connectors slightly to get them on but never much and never all the way around. I'm hoping it's something to do with the connectors however because the drivers are a much bigger problem. I use a Tubemagic D1 and I don't listen very loud so I dont think that could have blown the dirvers. However, my HD650s had the same problem a couple of years ago, but they turned out to be loose drivers that just needed put back into place. Any suggestions would be much appreciated as I have no idea what to do!
I don't really have an answer, but you may get better luck posting in the Headphone sub-forum. This one is the Dedicated Source Components, so a lot of the headphone guru's may not be looking here too often.
Can you try a different amp? Is it possible you just got used to the bass? I don't find the HE500 particular bass-heavy anyways.