What exactly are you looking for in a headphone? What sound, closed or open, and for what type of metal? You see, every single person will have a different taste in music, I EQ all my headphones, including HD800 when I tested it. In your situation, the best advice is to go with LCD series headphones, LCD-X or LCD-4, but they are heavy when worn.
I would any day go with HD600 or HD800S, I like HD600 considerably more than HD650, this is why I said that every ear is different.
Given that I heard most headphones from most price areas, if you want an upgrade from HD650, but if you want to keep the same signature, go with Sennheiser ie800, LCD-X or LCD-4, but if you're ready for another signature, go with HD800S.
@Music Alchemist is right when he is saying that HD800 has a different sound, they are much faster and more precise than most headphones, not to mention the soundstage size and instrument separation. The speed of each instrument is incredible and if you need a headphone that will truly make your music shine, it is your best bet.
In the meanwhile, ie800 is my IEM of choice at this moment, they have most qualities of HD800S, but with a more life-like signature, their mids sound livelier and more enthusiastic, where HD800 sounds more serious.