Well, I have bought a near mint pair of PM-1's to replace my PM-2's ... which look like a car crash due to the faux leather flaking off on the headband ....
I was expecting the sound of the 'new' PM-1's to be exactly like my PM-2's but it isn't !
Both have the 'alternative' leather earpads (as I bought a pair for the PM-2's some years ago) ... yet the PM-1's seem noticeably clearer than the older PM-2's !
More definition and tightness, clearer treble ... yet the should be the same ! The basic 'signature' is there but definitely cleaner, clearer on the PM-1.
Now, I bought the PM-2's in 2015 so they are 6 years old and well used. The seller indicated he originally bought the PM-1's were from 2018 .... did Oppo make any subtle changes in that time perhaps, or is this just down to the tiny differences between 2 pairs you may get ??
To me the PM-1 is a better sound, cleaner but still relaxed and listenable for hours on end.
***Edited to add I have listened straight away / back to back as both pairs can be plugged into my Chord TT2 at the same time***