"Comfortable" is not a word that comes to mind when I recall my ATH-1, which I sold or gave away many years ago. It was my first isodynamic and the first I tried to modify. The ATH-2 uses the same driver, but with a headband made of metal and thus adjustable for clamping force, it's much more comfortable.
The KSC-75 will be much more efficient, and unless the ATH-1 is modified in the same way we showed with the ATH-2, it won't sound as good as the KSC-75.
Rule of Thumb: Unless you have the willingness to modify them, it's best to avoid any of the old isodynamic headphones except the old Fostex T50.
You might be able to use this information to bring down the price of the ATH-1 you're looking at. It's historically interesting and has great modding potential, but comparing it to the KSC-75, there will be no doubt that the KSC-75 sounds better, especially on portable gear.