The Centrance DAC Port HD is the only USB-powered DAC that I know of which can power high impedance headphones.
My HD600 gets way too loud, even with the measely 0.5V EU Apple dongle.
That makes me think that perhaps you're not sending your audio to the dongle at full volume, hence you need a dongle with high output voltage to compensate.
Anyway, the DAC Port HD can output 4.1V into high impedance loads.
There are probably 50+ dongles on the market with balanced 2.5mm/4.4mm ouput jacks, which can about match it at 4V into high impedance loads.
However, those will require you change the headphone cable to a balanced one, otherwise you'll only get 2V via 3.5mm.
If you have headphones with fixed, 1/4" cables, so you need something that can output high voltages even with single-ended 3.5mm/6.35mm output, then the only alternative AFAIK is the Apogee Groove.
It'll play even louder than the DAC Port HD at 4.6V into high impedance loads.
Seriously though. Look into how much voltage you
actually need to drive your headphones to your desired levels. It's probably way less than you think.
For example, using my HD600 I need about 0.06Vrms Peak for EDM and at most 0.17Vrms Peak for less compressed, classic rock. 4V or more would be insanity!