Your HD700 are 150ohms.
Certain solid state dac that don't have a headphone output can also be used, from their line rca outputs, so long as they have a volume control and their output impedance is 20ohms or lower, and give 2v or higher at full output.
You'll just need rca to headphone adapter, this is the way I listen to my HD650's and it sounds better this way, direct.
Cheers George
