I just sold something that would have likely worked - and sounded - great. An HPDac, combo DAC and amp.
USB-powered & signal, it has a non-OS DAC and a pint-class amp built in. Sadly, no longer available to build new, but if you search the forums, you'll likely find some comments about it (including mine for sure).
A great and convenient unit that can drive many different phones, includign Senn 650's, AKG 701's, and more. I only sold because I upgraded to the $500 Pico.
Good luck, whatever you choose! If you decide to look atthis, I'd suggest a WTB thread in amp or source FS forums.