I own HD600, 580's, several portables and a set of wireless for my daughter. I have no complaints at all about sound quality but I won't buy Sennheiser any more due to build quality. My 600's don't get a lot of use yet the foam in the head band has already broke down, the foam in both pairs has decentigrated, the wireless didn't last very long, I've had friends who have had quality issues as well. The market for good sounding headphones provides a lot of options these days to have to deal with this.
Prior to moving to Senn I had a set of AKG for probably 20 years, I think I eventually gave them away. I take good care of my stuff and expect it to last longer than the Sennheiser did, maybe not 20 years but a better showing than I got.
As expensive as the 800's are they shouldn't just be falling apart.