First, let me say this is the first really expensive set of headphones I've bought, although I wanted a nice pair for years. These Sennheiser HD598's were intended for listening to classical music. They sound great on jazz, everything sounds quite good -- up close and detailed, but for classical they just sound veiled, distant, muffled, flat. Everything just sounds far away unless I fiddle with my equalizer in my Foobar player. I've tried them on 5 different computers/laptops. I have a cheap Boosteroo amp which only makes it louder. I don't have a DAC, but I understood these hardly need one, and I can't see that clearing up the problem.
I've been putting music and pink noise through them for about 24 hours now, slightly higher than listening volume, as I've seen recommended. I think there's been some improvement, but I really can't see any amount of "burn in" making these headphones great, and I have two weeks until I'd need to return these. Ironically, where many people complain about lack of bass, I thought that was actually fine.
So I'm quite disappointed. I got them for $190 on eBay. I've had a pair of headphones for probably 15 years that I likely spent no more than $25 on which I think sound superior.