Here's one option...but not very inexpensive...
The Headroom company specializes in headphone amps, including ones you can carry around with your iPod. The Total Airhead or Total Bithead models might be what you need. I gave one to one of my sons and it is really not bad.
Headroom's portable amps
There are other small, battery-powered headphone amps available also. You may find a few names to look for if you visit head-fi.org.
I have the same headphones and two headphone amps - cheap and really cheap - the new FiiO 5 is $20 bucks at Amazon - looks exactly like an ipod shuffle - charges from USB. And some other nice one that was $60 bucks - probably not made anymore. I bought cheap ones because I need portable - I used them to drown out the dentist drill for some long procedures.
The one piece of advice I keep harping on - PLEASE use the LINE OUT audio from the ipod jack, NOT the headphone mini output when connecting to any amp. The headephone audio out was not designed to be amplified impedence and gain is all wrong. The line out will give much better results - plus you leave the ipod on min volume conserving battery. If you use the headphone output you'll be in the same place - having to turn the amp output all the way up and also the ipod output up really high. (wasting ipo battery) Problem - its really hard to find a decent ipod output jack cable - I've made my own.
the concept of a mini-headphone amp for ipod use is patently silly--it belies the whole concept of portability. as good as the grados may be, i'd opt for koss portapros for ipod use (like mortsnets above)--they're more efficient, and consequently will play louder. at 35-40 bucks, they're one of the great bargains in audio, plus they're comfortable and sound very, very good.
Hi folks:Hi, Cathy. The Nuforce Icon Mobile should give you just what you're looking for. It's $99, is very attractive, smaller than an iPod, has internal rechargeable battery, and functions as a headphone amp for iPods or as a USB DAC for computers. Available on Amazon. It even comes with a silicon rubber band to piggy-back it with your iPod.
Amazon listing including 8 customer reviews.
Thanks for the help folks, that nuforce product sounds great.
Special thanks to Edgejazz - I had no idea that I had a volume setting on the ipod - turned that up and that did the trick! This will be fine for the times when I want to listen in bed. Loves it when the KISS principle works!
I do have some Skullcandy ear thingies for when I want to be very mobile.