The Emotiva Mini-X is one of the most highly recommended amps for headphones on this Forum. You should not need another amp. See:
I have seen that thread.
But most people tend to say that it is too much for headphones like the HD650 (and HD595).
Most claim it only really works well for power hungry headphones like the HE-500, HE-6, or even the LCDs. They even mention that the HE-400 is too sensitive.
Since I already have it, I suppose I should get an adapter and give it a shot to see what I think.