In regards to a tube amp, what about the Bravo Audio Ocean, or Aune T1?
I love my ocean, works great with my hd600 and k7xx. Tons of power, I literally can't turn that sucker past 1/4 without it being too loud.
Aune T1 works quite well, it was recommended as a economical amp by @mikoss for my friend who has a K7XX and HD-650. I just provided her with some tubes so she could experiment with tube rolling on a limited basis.