lowly audio virgin here.
got a set of BNIB HD598s off kijiji.
debating if the schiit MM2U stack is worth it. I doubt it improves the SQ enough over my Xonar enough to justify the price though.
might pick up the hd650s later. but sooooo pricey in canada... like everything else that's worth having.
Basically correct, as long as your Xonar can get the volume up to the level where you feel it's loud enough, an MM2U stack wouldn't improve on SQ much to justify the purchase, it'd only allow you to go louder. Besides, the HD598 is quite efficient and doesn't require THAT much power to get to a loud enough volume.
Have you tried checking out used cans? They can be a worthwhile avenue to pursue as, from my experience, mid-tier to high-end cans are usually (but not always) well taken care of by their owners. The AKG K812, Denon AH-D7000, HiFiMan HE500, HE400i and Audio Technica ATH-AD2000 (amongst others) that I'd gotten used can pass for brand new units. Just a suggestion as used cans are pretty good when it comes to value-for-money purchases.