Mr.Pea,

I'm using a Rio Forge, about the size of a child's fist. It doesn't do a good job of driving headphones (don't even try to drive full-sized cans with it), but if that's a concern, there are some pretty good portable amps out there (the bithead comes to mind). For home use, I use a Grado headphone amp. It's portable, sort of, but I don't really care to explain to the security check what it is each time I board a plane.

With the noice-cancelling mechanism on my Senns, I have to believe that it also serves as a small amp for the phones.