From the seeminly neverending posts about the KSC-75, i do not believe that the HD201's will really be an upgrade from the KSC-75's in overall sound quality. Both are great, cheap headphones which sounds like they should have cost much more, but i believe the KSC-75's to be the better regarding sound quality.
i just got my HD201's on monday, and have been quite impressed with them. i've only given they maybe 20 hours of burn in, but i still think they sound great for the $16 they cost me. i don't own the KSC-75's but have heard them, and if the Senn's seem like they should cost 3x as much, the Koss phones seem like they should cost 4-5x as much, in my opinion of course.
the biggest thing is that the Senn's are closed. they don't isoloate too well, but its enough. i can still hear myself typing at the keyboard faintly if i listen for it ... but they seal you in a heck of a lot more than the clip-on style of the ksc-75's will allow.
but hey, at $16 a pop, i think they're worth a try ... even if you just wanna check them out ... for something different.
i love mine. they haven't dissappointed me. sure, i know there's better, and have heard better ... but i don't put the 201's on and think "wow, these suck" its more like ... "ok, i can get used to this"