I own the original PX100, this is my second pair after I buggered my first trying to rewire them for balanced. I freaked out because the PX100 original was discontinued by then. Luckily, Head-fier Grokit offered to sell his pair to me.
I just read the OP and I couldn't agree more. I tagged my PX100s as much better to the HD595. I also believed that it was a smidgen better than my HD650s. Inspired by his post I just did a shootout with my PX100 and HD650 properly. But first I run a rough EQ on the PX100s to sound a little close to the HD650 - like this:
The PX100s are better than the HD650s and here is how:
The HD650s have a more 'out of head experience' and I can isolate this purely to the fact that the drivers are indeed further from the ears due to the pad. That is the one and only advantage. This is how the PX100s beat the HD650s:
The mids, even though are a little closer due to the drivers being closer are more three dimensional. The HD650 is less 3D. That is also to say, the PX100s space out the midrange when many instruments are playing together better, giving each note a better space and air between competing sounds. The midrange is in fact a tad more detailed and smoother.
Why this is, in my mind is quite simple. The mids in the HD650 have a cavern to deal with, the space between drivers, ears and ear pads. This leaves enough space for 'bouncing' of the sounds and muddling it up a tad...this seems really obvious to me.
The HD650 does handle significantly more volume without introducing distortion...but up to moderate levels - the EQ'd PX100 sound simply better to me than the HD650 - more resolving and more depth to the sound even though the 'headstage' is actually smaller...the 'soundstage' is bigger.
Believe me, I have known this for a while now.