Best headphone amp under $400


Hi all, what's the best headphone amp under $400 for B&W PX7? source is tidal (uncompressed) audio streaming. would like something portable. Audioquest cobalt/dragon seem to have good reviews but I am not familiar (had some cables made by them, they were good)

thanks 
speedmaster20d
I am looking for AMP + DAC in one package. I have read very good reviews of the AQ dragonfly. was looking at cobalt. DO you mean barely enough for high impedance headphones or just any headphone ?
thanks  
My Dragonfly Red is good enough from my droid phone to Tin hifi T4 iems  but connect to hifiman he-4xx and it runs out of steam well before I do.
Slightly over your budget, but you should consider Chord Mojo. It's not ultra portable, as in you can't put it in your pocket and go out for a run, but certainly something you can take it with you on trips. You might even find a used one for under $400.
thanks all ended up with dragon fly cobalt, sounds really good. had seen mojo pretty nice too but a bit too large (want to fit in a pocket without feeling it!)