I have the same problem!
I'm thinking Fostex HP-A3.
Why? I'm using the Yulong U100 right now, it's too forward, digitized, flat-fielded, and bright.
I asked Mike at headfonia for a comparison to the Fostex HP-A3.
response "Hi Nicol,
The Yulong U100 is brighter, more forward sounding. The Fostex is more laid back, smoother, more spacious, warmer."
I replied "Thanks![...] does it have any competition at the $400 level? Size is not a concern to me."
response: "The HP-A3 is very good for the price, but it sounds like you need a warmer sounding box for the A2000X.
Try the HRT MS2/MS2+ paired with the Schiit Asgard. Should be a better combination for the A2000X."
I'll probably go for the Fostex anyway, good for the price, a good DAC, 32 bit, and a quality product made in Japan, just like the A2000X.
However, like you, I'm also considering the Matrix M-Stage, perhaps with the HRT MS2+? Not sure.
You can also consider the Fostex HP-A7, if you can afford it.