Both headphones have mahogany undies and aluminum coats. The PS1000 has a larger aluminum coat. It also uses jumbos instead of bowls. Less visible is the fact that the PS1000 vents four holes while the HF2, at most, vents 1.
I owned the PS1000 and the HF2 at the same time. Unamped, the PS1000 had more thump than the HF2 and more HF extension. When I swapped out the bowls for jumbos, the HF2 had what sounded like the same HF extension but the bass wasn't quite prominent enough to remain full without the aid of a strong headphone amp. I remain convinced that the HF2 would have the same bass if you vented the driver further.