I've had, and loved Stax. It's about the best I've heard, in 15years of serious headphone geekery (A dem of the Senn Orpheus left me cold...)
That said - because I no longer use cans much - about the best commercial amp I've heard (or bought) is the old McCormack Micro Integrated Drive.It's basically a small power amp, good for 2-3W into 8ohms. It also takes hold of Sennheiser cans firmly and gets rid of their one downside - the bloaty, lazy bottom end.
The only other thing that got this good for dynamic headphones was a modified Quad 405/2 (!)