I've put the comparison on my list to try to remember to do tonight, but I think I can tell you my overall rating right now: KSC35 > DT231. I do not consider the DT231 an upgrade.
What the DT231 is really good at, though, is sounding nice with the volume way down low. There's enough bass and enough extended treble that you hear both bass and detail in a very pleasing way at quite low volumes, where other cans might just sound soft and indistinct. It's kind of cool, a very nice "background music" can. But unless you really have that need, I don't know of a good reason to buy them in preference to something else.
I think you'll be hard-pressed to find a closed headphone much under $200 that will beat the KSC35, at least unamped. With a good amp, I like the K271 a lot, and prices for it seem to be depressed these days.
As for the DT231 I believe it did improve a little with an amp, but only up to a point. Since it wasn't really meeting my needs, I didn't do a lot of experimenting with it.
I like the HD25SP enough to make it my at-the-office headphone. Nowhere near as much bass as DT231 or KSC35, if that matters to you, but a good sound nonetheless. The HD25SP sounds really quite nice out of a 9V/8620 Pimeta, pretty OK out of a Cmoy-level amp, and decent out of a cruddy unamped soundcard (your mileage and concept of decency may vary). Only with a Pimeta-level amp or better would I call it an upgrade from the KSC35 (and for that price you could get an HD25-1 instead, which I haven't heard but many people love). It has a useful amount of isolation, but with music off I can still hear loud-talkers a cubicle or two away.
For real isolation the (non-canal) phone to look at in the $100-or-less price range is the HD280. For me the sound is not right and the skullcrusher effect is too much, but many others like it a lot.