Mostly just posting this because I had to hunt a fair bit to sort this out. If the foam earpads on your Sennheiser PX-100s are shot, you can replace them with these guys from RadioShack (which are half the price of the replacements from Sennheiser):
RadioShack Replacement Foam Headphone Speaker Pads (Medium) : Headphones | RadioShack.com
They won’t *look* like they’re going to fit, because the skirt on the back of the pad is tighter than what you’ll see on the blown-out foam earpads that came with your headphones. But, if you slip one edge of the RadioShack replacements on, and then work carefully around the perimeter (kind of like getting a bike tire back on the rim), the skirt will slide right into place. The RadioShack pads feel good as new, if not better.
Incidentally, the PX-100s are *excellent* headphones (I don’t know if they’re $100-excellent–that’s the current price on Amazon–but were certainly worth the $50 I paid for mine, like, a decade ago). They are light, easy on the ears, fold into their own case–and thus travel well in your computer bag–and the sound is really nicely balanced, with good bass and clarity. The surprisingly good sound is a consequence of having an open-backed design, which means they function more like stereo speakers than headphones (important, since most music is mixed with actual sound systems, not dinky headphones, in mind). Most headphones are locked into plastic housings, which constrains how much air the diaphragms of the speaker can move. This fundamentally dicks with how you are hearing the music (since that’s all about vibrations in the air); the fact that the music is mixed to be heard on actual speakers doesn’t help. The result tends to be dead bass and muddy sound. An open-air design better emulates a speaker situation, with the added benefit (in my opinion) that it *doesn’t* block as much ambient sound (being sealed inside my music tends to creep me out; I have issues). Also, in my experience, and open-air headphone speaker is easier on my inner ear and ear-drum, since it doesn’t focus a blast of pressure waves into my skull.