Since there are already a lot of good suggestions on this thread, I thought I'd give an "out of the box" idea: A vintage, mid seventies receiver! With your budget, you could get a really nice Marantz/Harmon Kardon/Sansui/Sony/Pioneer receiver. They look amazing when fully restored with LED kits and would match your Sennheisers from the 70s perfectly! All you would need is a Schiit Audio DAC and you're in business.
I have a top of the line system, but my integrated amp doesn't have a headphone jack, so I'm using a Tandberg TR-2045 from 1978 as my headphone amp for the rare, late-night listening seshes and it sounds phenomenal (I use Sennheiser HD 600s).
Just another option.
Good Luck!