Let's see... here's my experience:
ES55: up-front and light-weight presentation. Some upper mid accentuation, so it's quite sibilant. Common problem for most headphones so that's actually "normal". These are "natural-sounding" to my ears. By "natural", I mean it presents the recording very similar to the way I hear it in real life. Good microphone required, of course.
ESW9: warm, thick, full sound. Decent bass impact, but bass blends in with low mid so it's kind of hard to distinguish. Overall still a light-weight presentation. Treble does not extend that high, but it's only slightly behind ES55 in that regard. Midrange is absolutely smooth and lush... with some upper mid accentuation. It's slightly less sibilant than ES55 and more enjoyable overall. It's a more "laid-back" sound to my ears. Not necessarily "natural", but I'd grab the ESW9 if I want to sit back and relax... with cool air breezing in.
ATH-900X: very light and airy presentation. Soundstaging is huge... almost rivaling the likes of DT880 / HD 650. Bass has nice impact, but nothing to really write home about. Midrange may sound thin and veiled... especially compared to ESW9. Overall, it's like a lighter and airy ES55. I think the 900X is good for some gaming and movies, since the light presentation allows a more effortless listening experience.
ES10: polar opposite of all of the above. First thing that jumps out to grab you is just how much bass there is in this little headphone. In fact, forget comparing it to those headphones above. I think The ES10 has enough bass (in general) to rival the likes of HD 650, D2000, and D5000 in this price range. The impact that you can "hear" in the ES10 is really headache-inducing after just a short while, and it seriously makes the need for any EQ or "bass boost" redundant. Having this much bass is not necessarily a bad thing, but sometimes you just want to have a light and relaxing experience. The ES10 won't give you that. It'll bash your ears into your skull if given the chance. Funny enough, midrange and treble are very good as presented by the ES10. It has just the right amount of accentuation for both so as to not let bass drown out all other sounds. I think the ES10 gives the most "realistic" presentation... even though it's not "natural" (there is nothing "natural" about having your ears bashed into your skull). By "realistic", I mean it sounds "as expected" most of the time, and then it always adds just a little pinch of coloration in to make the sound somewhat... different. It sounds "nothing like the other headphones".
My personal favorite is the ES10, of course. But that doesn't depreciate the other headphones. I'd likely pick up any other headphone depending on the occasion. I just pick up the ES10 more because it has this... "unique" characteristic to its sound. But to be more technical, the ES10 has better instrument and vocal separation than both the ES55 and ESW9, so things are easier to pick apart in the ES10. That's probably why I grab it most of the time even though I know it sounds "colored".
Haven't heard the W1000X or 700X to really say anything about them.
Edited by Bill-P - 10/27/12 at 10:54am