Have a look into the Audio GD NFB 5.32.
32 bit dac, Usb, Coaxial + optical. Delivers over 2 watts to HE500, more than enough for any headphones bar HE-6/HE5LE/4/K1000. Rather musical with filters to change the sound a bit, ACSS as well as without, changeable OP amps, can use purely as a dac as well. Has a good volume knob, which is welcome as many amps don't.
I personally have the NFB 28 and find nothing to flaw with any of my wide range of headphones.
Mind you with SS amps I hear only a small difference between them, if at all sometimes. I'm no expert with amps really - take it with a grain of salt
NFB 5.32 has been replaced by Compass 2