Might want to check this out. I use the Fii0 E12 its not the smallest but its very light and can easily fit in a pocket. Also, I havent noticed it heating up after extended use - a common problem with portable amps.
might want to try out the AKG K490NC's, I found them on PricenFees. They're relatively inexpensive, and they do the job nicely. I travel quite a bit for my job (2-3 flights per week) and have never had any problems.