My guess is I'll probably draw a few flames for this, but what the heck...
Before you go out and spend serious money on a pocket amp, try the FIIO E5 -- yes, really!
I got one for my iPod Nano 5G to drive HD600 phones. It sounds just fine, and for the money, how can you beat it? Just for grins, I did a blind A/B test between that setup and the iPod, my Home Headroom, and the '600's. And, yes, I can tell the difference.... long as I am playing lossless files ripped from CD, or VBR files. With MP3, no audible differences.
However, The FIIO sounds pretty good -- not as crystal-clear as the Headroom, but good enough that I often use it at home, as well as portable, simply from laziness not wanting to bother hooking up the iPod to the Headroom amp. There is no background hiss or noise, either, despite some reviews to the contrary.
It drives the HD600 cans well -- I set the FIIO volume up all the way, bass boost off (not needed with the HD600) and control volume with the touch wheel on the iPod. There's more than enough drive for anything I've listened to so far.
For $20, it's definitely worth a try. Having listened to a bunch of the $100-class "Altoids-tin" amps, I can say it's just as good as those.
Oh, yes -- it's small enough to stick on the back of the Nano with a couple strips of velcro, resulting in a combined package with the same outline as the Nano, and about twice as thick. The whole thing fits comfortably in my shirt pocket.
