Need Help In Discovering My Ideal Headphones

Sorry if this is a repeated/useless thread, this is my first thread as Im new. I have tried to do a bit of research before posting but due to my lack of knowledge on headphones I am still clueless. All I know is that headphones can be better than earphones, hence me wanting to get a good pair.
     I do not have a budget really, as I am and will continue saving up to get some headphones but going hugely over £200 is a bit too much I guess. I listen to mostly rap/hip hop and R&B. Also I listen to a lot of soul, ballads ... that type of stuff. Sometimes also dubstep, dance, pop e.t.c. Judging from this I'm guessing I need headphones that are pretty expensive as they cannot lack in low's, mid's or high's. I think I'd prefer the high to be crisp rather than dull, and pretty good low's so the bass doesn't dominate the music unless it should do.
        At the moment my setup will just be a cowon s9. I will be using these headphones a lot at home, but also when travelling to school so I'll need portable headphones. After reading a few threads I've come to understand I should buy an amp after buying headphones - so if you have any suggestions for portable amps that will go well with the headphones please say. I'm looking for a possible portable amp as well because after reading a few threads I think that despite cowon s9's superb sound quality their bass can be better and a portable amp can still improve the sound. I don't understand the setup of amp's but again have read that cowon doesn't have a line out - but that is irrelevant as a amp can still be used for its purpose.
      Sorry if I have got things wrong or if I end up asking loads of questions - I know it can be frustrating for you.

Thanks for the reply. I think the Denon 2000 is out of the question partly because of the price and also because some reviews have mentioned it does break easily after some time, meaning it would not clever to use them as everyday use - portable headphones.

At the moment it is between the KRK KNS 8400 and the Shure SRH 840. They are of similar price and it sounds like both can handle my variety of genres. I have some questions for both:

If i bought the KRK or Shure headphones should I get an amp/would a amp improve sound quality? Bearing in mind I will be using these headphones with a cowon s9.

Will they last for a long time, if used frequently at home and outside (on the bus, at school e.t.c)?

The only problems I did find was with the shure headphones - that they need to be burned in (which I'm guessing is normal for all headphones and not a problem, though I would like to be clear on how long I would need to do this.) and that they got hot after a long period of listening. On amazon a review stated that after a hour the headphones should be given a break, even if for just a minute.

Are there any other problems with the shure or KRK headphones?

Do you have any tips with how to maintain them? e.g. what not to do and what to do to ensure they are safe when bought, and last a long time without damaged sound quality.

I know this is a bit of a silly question but just to check: I can vary the size of the headphones? I'm saying this because I do have a small head and don't want to buy headphones that will just be too big for me.

Thanks for the advice. Looking at reviews I've seen people say they don't need an amp, but other people say a amp does improve the sound quality even with the cowon s9 as the source. Would a portable amp improve the sound quality? I am bearing in mind that the headphones don't need it but if the amp does make a difference then I think I would buy it so I get the most of my equipment.

Note: Im thinking of getting the KRK KNS 8400.

Thats made a lot of things clear for me now, because I know nothing about amps. I've just read two sides to them - they increase volume and they improve sound quality through channel seperation. Thanks and I will wait until I buy my headphones before deciding about getting an amp.

Back to deciding which headphones. The KRK KNS 8400's are still the no: 1 contender but the m50's are another good pair which is leaving me with a tough choice. When I do get the money (hopefully soon) I will just decide and buy :D