As it stands right now, I've been bit by the bug... and its awesome.
 
But... I am little limited.
 
I am currently driving my Hifiman HE-400's and Beyerdynamic DT-990 Pros with an Audioengine D1 DAC.
 
I keep waiting for reviews for the Valhalla 2 since I hear some nice things about Schiit products, but I've also been presented with an offer to buy an Asgard 2 AND SYS for a decent price, slightly used and with box.
 
My question is this:
 
Which should I lean towards that will also allow me to upgrade or get some other sets of headphones (Mad/Alpha dogs, LCD-2's) and still enjoy the ones I have now?
 
Any other suggestions? 

You need power. Asgard 2 is a good choice. Valhalla 2 won't do well against low-efficiency low-impedance phones like Mad/Alpha dogs and LCD-2.

Audio-GD NFB-11 has a ton of power. Great DAC, too. I recently upgraded from an ODAC/Asgard 2 and am so glad that I did