Well, it's been a while, but I've now listened to the k240, k271, and koss dj100 each for about two to three weeks. The first thing I'll say is that none of these really impressed me at all.
K240: The one I disliked the most. Cheap sounding bass that is flabby and weak. The mids are absolutely nothing special either- they basically fall between recessed and balanced, and sound peaky and unpleasant in the upper midrange. The highs are actually a bit too harsh for me on these also.
K271: These are easily the best of the three, but still not really what I was looking for. The k271 sounds MUCH more clear and clean when compared to the k240. When I amped them through my fiio e9, the bass was actually really nice. For the most part it was just sub bass, but it was deep, thick, and tight. The mids, while more clear than the k240, are still really sharp and peaky for some reason. They're not sibilant in the midrange, but they do have a strange edgy quality to them, which makes it annoying after a while. The highs are the best of the three however. Very clear and not too extended. The soundstage isn't as good as the k240, but neither of them are particularly good at soundstage anyway. Another thing to add, this headphone is easily the most detailed out of the three by far.
Dj100: All around better than the k240 in every area except soundstage. The bass is decent, the mids are smooth but boring and can get pushed back by the bass at times. The highs almost perfectly fit what I was looking for though. They're very non fatiguing, but a little more is probably needed because the mids sound muffled at times. My major problem with these is that they sound boring. With these I forget that the music is even playing after a while.
I think I'm more disappointed though because after trying all of these headphones that people tend to recommend for midrange, out of all of these, my little koss ksc75 by far have the most forward and enjoyable mids I've heard on any headphone I've owned. It absolutely boggles my mind that if you spend 100 dollars you can't get a headphone that has a good enough midrange to justify buying anything above the ksc75...
I have a friend who owns a pair of Senn hd598's, and after listening to them, I was absolutely blown away by their midrange. I find it really strange that in order to get an upgrade that large from $12 headphones, you need to literally spend $200.