Sorry for my first post being a cry for help rather than a useful contribution, but you have to start somewhere! 
But I will try to explain my needs as thoroughly as possible. Excuse me if I'm using not too accurate adjectives or if my whole story is not very well told, I master two other languages better than English. I've read the "head gear" section and some topics (especially this one by "eucariote" was very helpful), but thought it would be better to get some new ideas specifically for my needs.
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SKIP THIS PART IF YOU WANT TO GET TO THE POINT
First of all, I don't have much experience with full sized headphones. I used them in the past a lot, but never did I actually consciously "listened" to them really, because of different reasons (the whole situation, no time, no comparison, did not think about it, etc.). I don't have much experience with hi-fi either. And I don't possess any fancy gear.
However, I think I do have a good ear. I'm a great music-enthousiast, I play(ed) the guitar in a lot of different bands, have a little bit recording experience, my family is pretty much into music (I think I can call my father almost an audiophile). I don't know anyone else who is nitpicking so much about sound. I can imagine though, it's almost never good! 
Considering guitar sound, I think I tried most of the woods (of different parts of the guitar)/models (of the guitar)pickups/amps/speakers/boxes/pedals/etc. What I'm trying to say is that I think I have a pretty good ear despite my lack of experience with headphones.
Anyway, what I do have is a '09 Macbook (it wasn't labeled "pro" then, but it's the unibody aluminium design), an external soundcard (Line6 Toneport UX2), and a Sony receiver with a small 5.1 set (which I mostly use as a 2.1 because I can't get the sound of the vocals right in 5.1). So it's basically this:
Laptop---(USB)----> soundcard----(coax)----> receiver
> speakers
I would like to plug my future headphones into the soundcard. However, I'm not sure how well it can power headphones, I can't find the specific output impedance (I think that's what I need?). And the soundcard is USB-powered, so I would think the output is not great. On the other hand, it's a recording sound card with a dedicated headphone output with it's own output-potmeter - it should be alright, right? But then again, it's not the best soundcard either ($200). And that it's USB-powered is bugging me. Of course I could use the output of my laptop or of the Sony receiver. Maybe the Sony would do a better job compared to the soundcard?
Getting back to the headphones, I think I have one thing I can compare it with a little which may help you. Besides the (I assume pretty cheap) Sony open headphones I have (I don't like them much because they are not closed, the sound could be a lot better, and something inside the left cup started to rattle one of these days) and the Sennheiser HDR 115 wireless headphones (I don't like them because they are not closed, uncomfortable (on the air), the sound is mediocre, and the radio hiss is pretty annoying), I have the Etymotic HF2 (which are basically the same as the popular ER-4Ps I understand) in-ear phones. Which I totally like! It is well balanced, clarity is great, very "accurate", and offer great isolation and comfort for me.
Why I'm saying this is that I read a lot about them and comparable in-ears (also here on the forum), and the main criticism was (if I recall it correctly) was that they very have little bass, are not warm, too "sterile", are not so exciting. I think I can understand that, but I think I see it the other way around. I call all these things "accurate" (again, sorry if I'm using strange adjectives). Rather than a pleasant big sound, they confront you in the face with all the instruments separately, and are unforgiving for small mistakes. I like that. However I agree the bass indeed is too little, and overall they sound a bit harsh. I imagine in-ears sounding warmer without sacrificing in clarity and "accurateness" I'm talking about.
So I don't know if this says much too you. Bottom line is that I like "accurate" (I really feel handicapped in my vocubulary...) sounds, the bass doesn't have to blow me away, and I love the hear all the instruments independently. And of course I want to hear especially the guitars as good as possible. But still, the sound has to be well balanced.
What do I want to listen to with the headphones? To start with the less important things and go up: Movies. I love movies a lot, but I think I will be satisfied with most decent headphones. Music is far more important. What music then? All kinds of music (except metal and 99% of the R&B/hiphop (I even don't know the difference between the two)). Pop, classical, jazz, folk, acoustic, electronic. But what really gets me off is (hard) rock. Most of it is sadly from the previous millennium. Deep Purple, Pink Floyd, Led Zeppelin, Rainbow, Scorpions, Whitesnake, Black Sabbath... that kind of stuff gives me the goosebumps. So that's the most important.
Soundwise not less important however, is that I want to record with it. Also, I don't want to hear my acoustic guitar bleed through the headphone, I only want to hear the processed sound of the drivers of my headphone. It would be great if it could block my electric guitar too, but that's probably too much to ask. And although I'm a lousy singer, I think I want the same for my vocal recording - I want to hear myself as much as possible through the headphones only.
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Long story, but I think I've covered everything. To summarize it (also for the ones who skipped the part): I search for headphones with good isolation, well balanced "accurate" sound with much clarity suitable especially for listening to rock and recording. And that it will sound good with my setup too of course (which is an USB-powered soundcard or a not-too-fancy Sony receiver).
This is what I found so far for about $200 and with a not too high impedance:
- Audio Technica ATH-A700 64Ω
- Audio Technica ATH-M50S 38Ω
- Fischer Audio FA-003 64Ω
- AKG K271 mkII 55Ω
- Shure SRH 840 44Ω
- Beyerdynamic DT-770 250/80/32Ω
The review I mentioned above covers the last three, which is in favour of the Shure. So the logical questions are
- What do you guys think is the best for my needs (compared to the other headphones)?
- Did I forgot other headphones which are worth mentioning?
- How will they perform with my soundcard and/or simple receiver?
I think that's it. Enormous thanks in advance! 
Don't adapt your answers to it, but I think I will be too weak to resist to the temptation in buying a dedicated headphone amp. I've done no research in this yet, since I really want to use the above mentioned setup for at least the first couple of years, but could someone help me in the right direction in choosing an amp it would be nice. Again, I have zero experience/knowledge in this area, so a link to general info and comparisons would be the best I think.
