I've had this experience when pairing the HD650 with equipment that doesn't suit it. It can become very slow and dull.
But when you pair it correctly (whatever that is) and give it a bit of volume, it will really rock. HD650 does react surprisingly well to faster sources and amps and it will climb that mountain. I'm listening to Nightwish at the moment and it plays it great I think. z
Just wondering what you consider makes an amp or dac "fast"?