I'd suggest the Koss UR-55 over the Porta Pros any day. They are far more comfortable and sound much better. I don't think they have the same drivers because they have a lot more bass and it's nowhere near bloated. For me they're very good for rock music. The comfort is also good. Since I wear glasses, the Porta Pro's were painful on my head. Sadly, I lost them. I don't know how someone can lose a pair of headphones!
People who love the KSC75 and Porta Pro's REALLY need to try out the UR-55's. Strangely, these even sound better with an amp, but don't require one.
Oh and they're only $50! I consider them an upgrade of the Porta Pros!
Does anyone know anything else about the UR-55? Are they new? Are they a Best Buy exclusive? Right now they're kind of a mystery to me. I don't know if they're a re-release of an older model or what!