I asked this very question awhile back.
The verdict was the Amphone is not worth it.
Also, Im told the new Amplug is better than the original.
I've had the SWR unit as well as a Tascam CD unit and did not like them with decent headphone (Sony MDR 7506)
Same unhappiness with using a Fusion or Rumble with these phones although the kill in a real studio on bass with no compression or EQ.
My next try will be with a small mixer but I'm not expecting much.
My issue is hiss, lack of dynamic range, and the tone has no fidelity or punch.
YMMV