I purchased it for my HD800s. The soundstage was great at first, but now it is very thin, only existing in the middle. The "left and right of the soundstage" is non-existent. It is all focused in the middle.
My set up is:
HD800 > Matrix M-Stage > RCA Cables > Nuforce uDAC > USB Slot
To clarify the problem, I plugged the beats by Dr Dre into a 1/4 jack from their native 1/8 jack and inserted that into the Matrix-M Stage: No soundstage. I then put the 1/8 jack of the headphones directly into the computer and the soundstage spreads to the left and right speakers. I then plugged the 1/8 jack of Beats into the uDAC and inserted the uDAC into the USB slot, the soundstage is there.
Beware when buying the Matrix M-Stage from ebay. It can mess up easily.