It can make HD600s so loud you'll be crying with pain, if by "adequate" you mean "loud enough."
Even with a great amp, the sound out of the 96/8 PAD is grainy and harsh up top, with an occluded midrange, decent imaging (lots of blurring), and a rolled off bass.
Compared to other sound cards it is a reasonable improvement, but if you are considering it as a complete all-in-one source you would be better off saving for a used or new meridian (the GO7 would be great) and a max. That's what I'd use to drive HD600s or their bigger brothers, the HD650s.
I own the RME and I really wish I'd have saved up for a more high end source or vinyl, that stuff knocks the socks off of digital.
Some people on these boards have had good luck modding their RME card; I may or may not do it considering the difference in tastes, but you could always give that a shot too if you have the same problems that I'm having with my card.