I have HD25-1, it's designed for monitoring/DJing on the field: It's small, light and very rugged, it has short and reinforced steel (but microphonic) cord. External noise isolation is excellent. However, the fact that it's supraaural and relatively high contact pressure (for isolation) makes it uncomfortable (at least for me) to have a long listening session with it. It sounds quite boomy (but hey, I like MDR-F1!
) and it lacks detail. The soundstage is collasped. Anyways, it's not something I'd like to use at home to listen to classical music, but I use it every day on the train, and under this usage condition, I think it's much better than MDR-EX70.
I've never listened to HD-25SP, but I've read from somewhere saying that HD25SP is even more boomy than HD25-1.
I think HD280 is a relatively new product, compared to HD25. They don't seem to be closely related in design. HD280 is circumaural, presumably more comfortable than supraaural design like HD25, but it's also bigger and heavier. According to the spec, you can slam HD25 to ear deafening level of120dB(SPL) but 102dB(SPL) with HD280. Both are monitoring headphones, but it looks like HD25-1 is more optimised for outdoor use. I haven't listened to 280 neither, so I cannot comment on it's sound quality, but so far I have read some positive reviews on head-fi (as kelly pointed out).