Pfr-v1 uses litz braid. Imo it sounds quite airy, bad bass, sometimes piercing, but acceptable if you use stock cable without extension cable. I won't recommend recabling to people because of difficulty and limited cable gauge (maybe 26-27 awg max without doing something crazy to the wires or driver housing), although if you are an expert or can get an expert to do the job, I think recabling makes a good improvement because the litz wire is very thin, individually insulated wire. HOWEVER, simply using no extension cable or a better extension cable will be good for the sound, not mandatory to do full recable to make the pfr-v1 sound more acceptable. Here is a comparison of one doublehelix nucleotide wire and signal+ground stock pfr-v1 cable with ONLY the black sheathing removed, to show how thin it is.
I think I could sum up the pfr-v1 sound quality in one sentence, good mids and highs, bad bass and mediocre lower mids.
It's difficult to compare pro 900 to pfr-v1 because they sound quite different, but I'll try since I have them. In terms of enjoyment AND faithful emotion simultaneously, the pfr-v1 certainly has the edge over all the incarnations of pro 900 I have tried, but Kees modded pro 900 can be so euphonic to put a big smile on my face while other headphones don't, but I value faithful emotion and dampened pro 900 has that reduced. In terms of pure detail, the pfr-v1 is maybe 75-90% of the pro 900 depending on songs, being weakest mostly in ambient bass, electronica and instruments in the bass and lower mid region. When I listened to undampened pro 900, I think "okay, accurate, that's cool but I want to hear something interesting" and get the urge to stop listening because it's so predictable. Dampened, pro 900 sounds enjoyable, but emotionally untrue for certain notes you know should have more decay/harshness, giving a slight urge to remove headphones after prolonged listening. Pfr-v1, even in stock form, makes you want to sit and listen to the music unfold until your ear canals hurt from the ducts (won't happen if you do padding mod) without fear of inaccurate emotional portrayel, never oppressive like undampened pro 900's analytical sound or too sugared like dampened pro 900's forwardness of vocals and quick instrument decay.
Pfr-v1 beats the hfi-2200 in all areas except in bass and surround sound soundstage, and differs in sound signature by being neutral while hfi-2200 tries to smooth all sounds. It thoroughly trounces the icans/zino except humming-type bass, looks, and portability heh heh. Somewhat better than my not-so-well-positioned $1000 klipsch speakers.
All this is my opinion comparing stock pfr-v1 to other headphones, mostly because I don't recommend recabling the pfr-v1 unless you are pro at equipment dissembling and soldering, or get a pro to do it. If I compared recabled pfr-v1 to pro 900, I'd say it is about 90% of the pro 900, beating it on a few genres with instruments meant to travel a long distance, being beat by pro 900 by a pretty big degree for insanely mixed electronic music, but still, after recabling, and being manipulated by my very fast balanced amp, the pfr-v1 can do some bass that makes me scared I will wear down the drivers, but not as scared as I am for my pro 900 trying to do bass like violins :).