If you want a good AMP for good headphones, do not go lower than ASUS Essence STX II's quality.
There should be better ones...even perhaps better and cheaper..but that STX 2 should be your lowest quality standard.
You can ignore my next blabber:
If money is really not an issue, I would start looking at Sennheiser Orpheus, it is a DAC+Amp+Headphone bundle.
It should cost only USD 55k