Yeah, I've got the Z-5500's, they're great. Huge difference upgrading from a $100 pair of Shure headphones. The bass literally rocks the house.
As for the Dolby Pro Logic, I just got the Razer Barracuda AC-1, and it's awesome. Huge difference from onboard sound, plus there's a ton of settings to tinker with to get the sound just right. It's actually got Dolby Pro Logic IIx, which can upconvert stuff as far as to a 7.1 surround setup.
Now that I think of it, I should post a review...