[BornReady]

Could anyone please comment on the synergy of these two amps with the HD650? Which one is best, or should I be looking at a completely different amp (max price is $300). Furthermore, how well do they work with HE400s? 

 

[fabio-fi]

Both amps work well with Dynamic headphones like the HD650. For planar headphones, they will be ok probably you need to turn the volume more than half to have reasonable volume levels. 
Note that the M-stage have a Dac, and the Asgard is a dedicated headamp. 

 

[Uberbob102000]

Both amps work well with Dynamic headphones like the HD650. For planar headphones, they will be ok probably you need to turn the volume more than half to have reasonable volume levels. 
Note that the M-stage have a Dac, and the Asgard is a dedicated headamp. 

The M-Stage only comes with a DAC if you get the M-Stage USB. However I'd avoid it as I've heard that it's rather mediocre.

Both of those should drive the HE-400 no problem as well.

 

[DanPluck]

Listening to my new HE-400 on an M-stage now, volume knob at 9 o'clock.