Right now I'm using an HD650 with Enigma Oracle cable. In a word, the amp sounds marvelous, easily exceeding my expectations. Smooth, detailed, powerful. Really brings out the sometimes hazy midrange of the HD650. Huge midrange, startlingly focused "center channel" (like the way vocals often are in the mix).
One of the first things I thought upon hearing the amp is how amazing it would sound with a balanced HP2. Of course it helps I heard a balanced beta22 with a balanced HP2 at Canjam.
I listened with my HF-1 single ended and it sounded great as well.