I heard the PS500 today. I definitely enjoy it a lot more than the RS2i that I had. The RS2i was a bit bright for my preferences and the PS500 was enjoyable in a similar way to the HE400 I used to have. It certainly had a nice feel for Doom Metal and other darker styles of aggressive music. If I were to ever pick up a Grado again this would be it, but the LCD2 and Mad Dog hold down the dark headphone catergory for me. The PS500 had a better soundstage than the other Grados to my ears, more relaxed and less in your face. Thick mid bass tilt which give a phat bottom end to guitars but didn't have the in your face upper mids and treble.
Not sure if I would prefer it to the HE400 or not, couldn't do a side by side having sold the HE400.