I've got the MD5000's, and I just heard the PS500's yesterday, compared to the HD800's and RS2i's. This was out of the Grado HA-1 AC, and the Dac Magic Plus, which may not have been the best amps with the HD800's.
To me... the PS500's are superior to the rest... without going into all the details. They just seem to do everything better than the others... with the possible exception of the soundstage of the HD800's - though the soundstage of the PS500's is great.
Now... the SN of the HD800's was around 2050... and... from what I've read... the current version is somewhat better than the earlier versions (better bass and highs). I just found the PS500's had a richer, fuller, more weighty sound... yet... still had just as much detail, resolution and refinement as the HD800's... and... a great soundstage, though not quite as expansive as the HD800.
I found the PS500's were superior to most of the other phones I've had, or heard - including, the O2's, SR507's, RS-1's, MD5000's, "new version" HD580/600/650's, HD800's, etc. The only phones, which I may think are equal, or better than the PS500's are the ATH AD2000's. Though, I should note, I've never heard the O2's and SR507's with an amp that really mates with them well (BHSE, KGSS, 717, or 727, etc.).
I was so impressed with the PS500's... compared to all the others... I may just pick up a pair... or... perhaps the PS1000's!
UPDATE: Though, I have to admit... having a well matched amp... makes all the difference in how any set of phones sound. Right now... I'd have to say... the AD2000's and MD5000's driven by the Red Wine Audio Isabellina HPA may sound every bit as good as the PS500's - but, then I don't have the PS500's to compare.
Edited by Gradofan2 - 5/11/12 at 10:32pm