The ESP/950s sit above the Sennheisers and I would assume the ATs as well (I've never heard a closed can that can challenge the Sennheisers let alone outright better them, and the 950s are better than that); I dislike all things Beyerdynamic so I'll leave that one alone. :veryevil:
The 950s are blindingly fast, intoxicatingly smooth, and beautifully presented - everything from bass to treble and nothing is overly bumped or offensive (although the mids are a bit present; it's not "in your face" or "romantic and syrupy"). They can dig down very low on the LF side as long as the material genuinely has something to dig (e.g. organ works, some rock, some dub, etc) - they don't have a lot of resonance/decay/etc like many headphones do so there isn't a lot of "ringing" or "boom" as a result. The E/90 works well enough that I'm not interested in upgrading (it's also the size I think a headphone amplifier *should* be, but that's for another day). It's easy enough to get an adapter or recable on the 950s to run them on STAX Pro sockets (from whatever) though; if that's your thing.
Having said all of that, the Sennheisers are probably the closest to them of everything you listed (and perhaps overall) - they're smooth, balanced, relatively fast, don't punch anything into your face, etc. They're comparatively slower and somewhat "mushier" (especially down low) and a bit darker, but for half the price (oh, grrr, forgot about the MAP!) they're very good. I'd gladly take them in place of the 950s, if there was something that precluded owning the 950s (I can't think of a good example) or otherwise made them impractical (but really both are about as versatile - the Senns are fairly efficient dynamic cans that can drive from a lot, and the 950s just need a line (ish) source and the E/90 accepts both TRS and RCA; they're both open and provide no isolation, and neither will survive mobile (ab)use so there you have it). Both are very comfortable, but the 950s, being larger, are slightly moreso (even with the non-fabric pads). The warranty on the Koss, as expected, is better; as is availability of parts and their pricing.
Regarding any closed headphone I've ever tried - they *all* end up failing at one extreme or (even worse) dead-center in the mids. I've not heard a one of them that can get speed/low-resonance, FR balance, PRaT, etc absolutely correct in all things against a good open-back can. Sure, there's probably some $1000+ closed-back that I haven't tried or heard about that might get it right, but we're talking $1000+ just to compete, not to improve. That seems a little backwards imho (unless you have an absolute NEED for a closed can). The closest was the Kenwood KH-K1000 and apart from being very heavy and very warm (to wear), the only gripe was the potentially (And only slightly) over-done bass. Otherwise they're a fantastic closed can, and get very close to being a "closed Sennheiser" - very comfy (apart from weight/heat) as well. They also cost less than the A1000X. I would still say that either the Sennheiser or the Koss are better all around choices.
Depending on how serious you/your brother is about listening, and what you're plugging into, I'd probably weigh the Sennheiser against the ESP. The ESP *are* better, but if you're not going to appreciate the (admittedly small-ish) gains (there's just lots of little things they do better, they aren't "3 times better" or anything like that), save the money and get the Sennheisers, and put the rest of the money into a nice night on the town or some other celebration. This isn't a "you aren't worthy" kind of argument - like I said, I could live with either, and honestly I think if you like one you'll enjoy the other as well; one just happens to cost $600-700, and the other $300-400. So if you're somewhat indifferent about absolute fidelity, save the few hundred bucks.
Quickly between the two:
ESP/950
+ Better sound-staging
+ Faster
+ Better bass extension and impact
+ Somewhat more comfortable
+ Slightly mid forward; makes vocals nice
+ Somewhat better up top
+ Lifetime warranty
- More expensive
- Upgrade amplifiers start in the thousands, not hundreds
- Some people don't like their look/build-quality (lots of ABS)
Sennheiser 600
+ Somewhat smaller
+ Better grip/clamp on head (they don't "flop off" as easily)
+ Cheaper
+ Build quality is probably better
+ Laid-back top-end and very forgiving overall - nothing sounds "bad" through them
- Less details
- Bass isn't as tight/controlled
- Spares cost more
Oh, I should also add, if you/your brother prefers a "darker" presentation, get the Sennheiser, if you like a "mids" presentation, get the Koss. I'm not sure what a "kingrex" dark can would be these days - the previous Koss ESP was dark (good luck finding one in good condition/finding one at all though), and they sound very unique as a result. But other than those, I'm not aware of anything that's head and shoulders better than the Sennheiser that's also dark (without being a bass cannon).
