Its worth noting that the PB1, still being offered on the iBasso site, is considerably cheaper than any of the other balanced options, including its more powerful PB2 sibling - not sure how easy it will be to hook your HD600s up to iBasso's connector.
The iBasso site seems to be down atm, at least here in Oz - interesting that they were hacked this time last year and had no web presence for over a week. Headfonia Mike has some good things to say about the PB1/PB2, even if its not his preferred sound sig:
http://www.headfonia.com/the-usual-suspects-12-portable-amps-compared/4/
I've never been attracted to these amps purely on the basis that I wouldn't be prepared to have single-ended phones recabled purely for balanced use - I believe that Sennheiser designed the HD600/650 to work interchangeably from the beginning, so you are at a distinct advantage.