Compared to the Koss SP540: I thought the two would be very similar, variations on a single model, but they really aren't. I got the SP540s and was really disappointed. They sound fine to me, but the fit is terrible on my very large ears and they just feel physically cheap. These Koss Pro4s on the other hand, feel very nice to me. They are heavier, and while that is clearly not as good from a long-use comfort perspective, they just feel more rugged and "nicer." I like bass and I'll even admit to being a bit of a bass-head. These don't do the bass emphasis that the SP540 and SP330 do, but I think the bass on these is really quite adequate. I don't feel like these need more bass at all. Actually these produced some bass notes that were so full that they had me wondering if I somehow had the subwoofer playing. That's pretty special and not something I experienced with the "bass emphasized" SP540 and SP330. So if you love bass, consider that info carefully. I haven't even had them an hour yet, but so far the sound is very much to my liking. The frequency response seems relatively flat so they aren't "exiting" right away, but I get the feeling that will be an advantage with time, as I expect these to be better for a variety of music styles, rather than just the one style of pop music to which the equalizations of trendy listening headphones lend themselves. These still don't go all the way around my ear the way the better Sennheiser over-ear models do, but they fit on the outer edge of my ear and are quite comfortable so far. The sound isolation on these is very nice. I really feel that these are a whole step, or maybe even two steps, better than the SP540.
For anyone who happens to find this review and is trying to decide between the two seemingly similar models, I think I would sum up the two new Koss over-ear offerings of 2014 as follows:
Koss Pro4s: ruggedly built, pro style, frequency response curve for critical but enjoyable listening, excellent isolation, medium over-ear cups for medium large sized ears but suitable as on-ear headphones for very large ears
Koss SP540: light weight, attractive, frequency response curve for fun listening, very good isolation, small over-ear cups for medium sized ears and smaller