Headphones for Rogue Cronus Magnum

Good afternoon. I'm starting to get a bit confused by all the choices in full sized headphones. I think this is because so many people use them with ipods etc. I'm looking for some headphones that would run about $300. I would be using them with a Rogue Audio Cronus Magnum integrated tube amplifier that puts out 90 watts.

I've got a fairly large head. Listening for 2 hours at a time would not be out of the question. I listen to jazz, a lot of it with vocals such as Cassandra Wilson. I also listen to blue grass, some folk and a little rock and roll.

I was serioulsy interested in the Grado 325si, but am concerned that I would get fatigued from the highs and also that they would be uncomfortable. I like the Sennheiser SD600's but they take some power to drive them which I guess I have. AKG's sound like they might be nice but that's a heck of a break in time and I don't want to put my tubes through 300 hours of use to break them in.

Any advice would be greatly appreciated.