I currently own the KSC35, 75, Sportapros, and the Parts Express headband (popular for the Koss clips ons).
Sportapro drivers = KSC35 drivers
Swapping out the headbands and clips, its obvious they are the same.
The KSC75 out of the same clips and headbands is still on the brighter side, so it should be noted that they aren't the same. Just wanna confirm these things that have already been said.
As mentioned by others here, the drivers are sensitive to distance and tightness, so depensing on what you have attached, they will sound different. I feel the KSC35/Sportapro sounds best out of the KSC35 clips, which maintain a good balance between bass, mids, and treble. They aren't bendable, so the sound stays the same, as does placement.
The KSC75 sounds best out of it's own clips as well. The KSC35 clips don't give the KSC75 enough room to move due to the bulkier rear housing on the KSC75. The sound is a bit darker and muffled due to how restricted the KSC75 is.
The Parts Express headband as well as the Sportapro headband are both relatively comfortable, but it's much harder to get a proper placement of the drivers on the ears. Also, because both headbands are bendable, sound quality can drastically change for better or worse. The Sportapro headband fresh out of the box is VERY tight, so it makes all the drivers very bass heavy and muffled in comparison to the others until you bend it open. It's not as easy to bend as the Parts Express headband.