[dailysmoker]

you really should go for the jvc rx700 the bomb for only 68 euro i get them but you can get them cheaper for 50 $ gr tim

 

[Tomahorc]

I know this is an old thread, but thought I'd add my experience with my Q701s used for this purpose.  I bought these because of a lot of reviews saying what a massive sound stage these have, + great detail, with my primarly use for them being plugged into my guitar amp.  I wanted detail so I can hear every tiny mistake or bits of sloppiness in my playing, and they don't disappoint in this respect, and I wanted a large sound stage so that the guitar wouldn't sound so close to my ears (which I reckon was adding to the earache problems I was having when  I used my other phones)
 
I use a Blackstar HT5R amplifier which I would say has a pretty good headphone out.  You certainly don't need an additional amplifier to power the headphones, the HT5 gives you more than enough volume.  I have backing tracks coming from my PC through to the amp, and these come through to the headphones so loud that I have to have the voluem on the PC at the very bottom, and also the volume on the media player at the bottom too.

What really astounds me is that I can wear them for hours and hours, without getting fatigued or sweaty ears.  These are my first openback headphones so maybe this is normal with this type of headphone, with the other cheap closed phones I used with my amp I would get terrible sweaty ears after 30 mins, and then within an hour I would get terrible ear pain in my left ear.  With the Q701s, no ear pain, no sweatiness, they really work well for me for guitar playing.
 
Whether these are worth buying solely for using with your guitar amp, I don't know.  I bought these when I was on holiday in the US and they were $260 (as opposed to £300 in the UK!), so I felt like I was getting a great bargain.  Although I used them loads for listening to music while I was on holiday (and I must say, they are pretty incredible for that!), I pretty much don't use them for anything other than playing guitar now that I'm back in the UK.  A waste...perhaps, but they really allow me to play for so much longer than I could with my other phones so for me it's worth it. 
 
So in short, my advice is, go for open back headphones unless you really need noise isolation.  Look for ones which are reputed to have a large sound stage as imo, that helps minimize ear fatigue.  Go for ones which are regarded as super comfy, so you forget you're wearin em and finally, if you really want to help improve on the little things in your playing, get phones with lots of detail in their sound.
 
Hope that helps.