Hi Dan
For the Sennheisers I have I really like the Grant Fidelity DAC-09 with an upgraded Western Electric tube. I used an RCA to headphone jack adapter on the tube output section of the DAC-09.
Other people I know who have the Sennheiser HD-650 really liked them with a Schiit Asgard headphone amp.
Good luck with the search.
For the Sennheisers I have I really like the Grant Fidelity DAC-09 with an upgraded Western Electric tube. I used an RCA to headphone jack adapter on the tube output section of the DAC-09.
Other people I know who have the Sennheiser HD-650 really liked them with a Schiit Asgard headphone amp.
Good luck with the search.