TH-900 is very efficient. Any Android, Apple, etc device should power it sufficiently. When I'm lazy, sometimes I'll grab my iphone and the TH-900. Doesn't sound as good as my home rig, or my portable rigs, but it is good enough and still better than almost anything I've heard as far as full-sized headphones go.
Okay I was unclear there. What would be the bare minimum to get it to sound better than a lcd2+lyr+bitfrost combo?