Someone earlier brought up Audio-gd's
Precision1 - 16W @ 40ohms, ~$600 - as a step-up candidate from the Mini-X.
Since the Mini-X is 15W @ 50ohms, power is pretty much a wash, but priced <1/3 of the Precision.
Kingwa makes solid stuff, no doubt about that. I've owned a few of his amps. Both Emotiva and Audio-gd lean more toward the "industrial" design aesthetic vs. the very very pretty Krells or McIntoshs of the world (IMO).
So, in thinking about "what's next" above and beyond the Mini-X, for me, I'd be looking for refinement without sacrificing power. Given the extreme bargain proposition of the Mini-X, this is a tough call.
To my ears, as much as I'm absolutely loving the sound generated by the Emo, she's definitely a forward, non-subtle beast. I liken it to listening to the seeming limitless power of a concert / PA speaker vs. the liquid coming out of a Paradigm Signature mid and tweeter.
I once heard Oleta Adam's "Everything Must Change" on a set of $50k McIntosh speakers, and it was one of those reference point changing, revelatory experiences, as to how sound could "sound".
As Koiloco said above, do we have to go 10x (above the Emo) to reach that point where the SQ improvement / delta is worth it?
Or do we move incrementally, adding other tweaks such as tubes into the audio chain?
Or, of course, change the rubber that hits the road and pick up a headphone that has epic synergy with the Mini-X, and will exploit its power to produce that lush, "Diana Krall is singing to me, only me!" experience.
These are just the late night ramblings of a very satisfied audio geek, who, through smiles and epiphanous satisfaction of what $169 can buy... is still on this godforsaken site, looking for the next best thing.