I just received a Go-vibe v5 portable headphone amp to put between my 4th gen iPod and Senn 600s and various in-ear monitors. All I can say is wow. You don't really appreciate how limited the built-in amp is--it's not terrible but by comparison it's a mess. Pretty much every aspect of the sound is considerably improved, and if you are using apple lossless there is a level of detail retrieval which does credit to redbook cds. Even in-ear monitors which are supposed to be ipod-friendly really improve with the additional current from the portable amp.
The amp design favors simplicity and sound quality over pretty much every thing else. It's cheap enough at $75. It is said to have a nice open and transparent quality, which I've just gotten little tastes of as it's still not broken in. The part that sucks is that a dedicated iPod-to-mini cable tends to cost way too much money for what it is, well over half the cost of the amp itself! Even together these cost less than the headroom airhead. You can then get fancy with outboard power, rechargeable 9volt batteries, etc. But the basic amp is a very enjoyable product to listen to, and if a duracell lasts the claimed 75 hours or anywhere near it buying a new one every once in a while works for me.
