The HD600 and HD650 are my all-time favorite headphones for the majority of my listening, too. The main weakness they have is distortion in the bass region, which can make them sound slow and muddy on my head-baffled omni recordings. I suppose bass distortion makes studio recordings sound more realistic, and I've read the latest HD800S uses the same trick to improve perceived bass response over the original HD800.
But I often prefer the original HD800 because their bass is better defined due to low distortion. The bass is not louder, but clearer than any other headphone I've tried. That works out great for head-baffled omni recordings I make. I don't find the highs or resonance annoying like many do: I suppose my sideburns tame the ringing; and I EQ the hell out of the highs on DPA 406x recordings regardless.
The HD800 is a lab microscope. An incredibly detailed tool for analyzing and spotting faults in my own recording and editing; but not something I would use to enjoy less-than-impeccable sources with.
The other great thing about he former/present flagship Sennheiser models is the availability of replacement parts. Not just foam pieces and cables, but drivers, carbon fiber frames, and grilles, too. Just call them directly.
But IMHO, the best bang-for-the-buck is still the $15 Koss KSC75. I use them when I'm away from full-sized cans and speakers. Bass is even more distorted than the Sennheisers HD6X0, but all the mids are right where they need to be. Wearing them under a beanie or headband improves the clamp and bass response. Lifetime warranty, too