When I listened to the HDP with my HD600s, they were warm to me with a musical "bump" in the upper bass/lower mids. The reason why I wasn't totally sold on it however was that my normal listening level in that store's hi-fi room was still at around 12:00 to 1:00, and on some audiophile recordings with lower gain, would be around 2:00-3:00. Normally that's not a problem - when I listened to the Meier Cantate.2 before that my listening level on low-gain setting was at around 1:00-2:00 too. However, the problem with the HDP was that the bump in the frequency response caused distortion at my listening level. I don't even listen that loud compared to most other people I know, and yet I got the HDP to distort in those freqs; it's like pushing a car audio speaker (as opposed to a standmount at home in a sturdy, well-designed ported cabinet) without a sub and you hear, at some point, loss in the deeper bass notes replaced by a "thwack" at every excursion.
My earpads were new though, so the other people who liked the HDP with the HD600 might have had the drivers closer to their ears and didn't need to push it that far. Still, while I might caution about expectations for the HDP, if I didn't come across a Meier Cantate for sale here I would still have gotten it and saved up for the upgraded PSU (or I'd have gotten one with comparable current and voltage specs from Elpac if it saves me a lot of cash), as it was the obvious weakness in the entire package (it comes with a netbook PSU I think).