The HE-500 is definitely the superior headphone between the two, as expected. It is more detailed, more resolving, has better imaging and sound staging and a much better sounding midrange. The PM-3 is a can with a slightly raised upper midrange and lower treble, the bass is above the neutral point - quantity goes to the PM-3. The PM-3 sub bass weightier than any other planar cans I've owned (PM-2, LCD2r2+LCD2F, HE500, HE6, Vibro, Alpha Dog, HE400i, HE400). Overall the PM-3 can sound laid back with some music too, depending on the recording balance. The PM-3 has great imaging for a closed back of the price, but soundstage is not very large - definitely sounds closed in sound to me. The PM-3 is resolving, no blurred details - but it is not going to compare to the big boys in this regard. The PM-3 is very portable, small and isolates quite well, no issues there. However I do not feel that it scales much with my setup - maybe some other amps might produce better results.
I tested all this on a NFB 15, NFB 28, HPA M1 and WA7 btw. For me the Audio Gd amps pair really well with all the Planars I've fed them with. YMMV of course, I could easily see someone preferring the PM-3 if sub bass extension and weight was a massive factor. I'll admit they sound rocking with stuff like Infected Mushroom, Noisia, Juno Reactor etc.
Thanks heaps! just the sort of info I was looking for.
I get that it doesn't particularly scale well, but I would use it only with a DAP and preferably without an additional amp. In any case it will not replace my HE-500 for home use.
It looks from the review like the PM-3 got the bass right compared to PM-1/2 as well.