I recently picked up an Audio-Gd C-2C which I got information about on the head-fi site (good tip, Nsgarch). It is neither portable nor tube which means this post is of little use to you. Nevertheless, it sounds terrific with the AKG K-702s and at the price $335 plus delivery, has been a revelation for me.
I can highly recommend soliciting input from the Head-Fiers!
I can highly recommend soliciting input from the Head-Fiers!