The HDP is a very good unit and it'll be pretty damn hard to beat it for output honestly. If you NEED to spend money, I guess you could buy a new amp to chain to the HD600 (it won't make a huge difference unless it's tube) or maybe even cables honestly. You have a very well-rounded setup and I know it's pretty rare for people to hear that what they've got is good enough on head-fi, but thats the honest truth. The HDP's funky dynamics are what make it so good, the output actually is pretty dang good too. You get a lot of punchy bass compared to most solid state amps and the treble only gets sibilant on lower impedance cans that require a lot of current.
I mean if you're really out to spend money, maybe try cables? I mean I know they carry a stigma but most cable manufacturers actually offer some sort of refund policy, but I'm certain that once you give them a try it's pretty hard to go back.