A heretical view on my part perhaps, but I don't feel a headphone amp is necessary for the Senns.
Most of the time I simply plug my HD650/600/700 directly into whatever is playing the music. Sure, a nice headphone amp can improve things, but not nearly as much as the headphones themselves and I don't consider use of one necessity for satisfactory use. They all sound great plugged directly into any of the audio recorders I've used over the past decade, including R09, R44, DR680, R88, and Zoom F8 (F8 benefits from selecting a higher headphone gain setting in the menu) among others, various stereo and home theater receiver/amps, as well as straight out of the built-in headphone jack of computers. Granted all of my 'phones are very well broken in / "loosened up" with lots of playing time.
If I really feel I need to hear the Nth degree of detail and linearity I'll pull out the Mytek DAC, but most of the time I don't bother and am happy with the direct simplicity.
Others may disagree, but this works for me and avoids complication.