Alright so I've had some time with both, and I have to say I find myself preferring the portapro. The PX is warmer to my ears, and while its presentation is more spacious and natural, I find the KPP more realistic sounding in terms of timber. Its also faster, which I like, and even though the PX seems to extend further on both ends, the KPP is more balanced and not as thick-sounding, with slightly more emphasis on treble out of the box. The bass emphasis on the PX seems more pronounced, and it sounds slightly veiled in the upper mids compared to the Koss.
TL;DR The KPP is essentially a Grado-ish PX; the PX is a Senn-ish KPP.
As a side note, I prefer the build and overall design of the PX, they look classier and are clearly sturdier and better engineered, and the single sided cable is much better, but I do like that I can wear the KPPs while biking because they have that ultra thin metal headband which fits inside my helmet, and they don't isolate like my IEMs, so I feel safe wearing them while biking on roads. The mic on the KPP is also better quality (KTC model) according to friends I've spoken to on both headsets. I can even talk while riding with the Koss.
If I could only keep one, it would be the KPP because I prefer the sound and like wearing them while biking.
Edited by Gilly87 - 3/2/14 at 2:09pm