I agree but would also suggest a good integrated amp too. My NAD C 326BEE makes my HD650's sing.
I've heard the NAD integrateds have nice headphone outs, but I meant cheap like 25-100 bucks to get your feet wet. My first vintage receiver was a 30 dollar garage sale find for 30 bucks. It was a Pioneer sx-550, a bottom rung 20wpc mid 70's unit. Hooked up my dac and plugged in my 650's and was like holy crap! It blew the doors off my Asgard 2 in every possible way.
Fast forward a few years and a bunch of craigslist finds later and I've got my end game peice, a Sansui 9090 which was totl in 75. I had it brought back up to factory spec by a Macintosh tech for a couple hundred bucks and its by far the best I've ever heard my 650's sound, giant three dimensional soundstage, strong punchy bass, clean detailed highs and a warm enveloping midrange. It also drives my power hungry Dahlquist dq20 speakers exceptionally well...