Picked up one of these a week ago and popped in a LM4562. Sounds great with my K701's, very good match. I really like the tone it has, quite refined and slightly warm at the lower end, also good airy soundstage that is welcome for classical music. I'd say its a better match for the AKG's than the Magni that I got recently aswell. The magni is a little too "neutral" for the ultra transparent K701's.
The m-stage is struggling with my LCD2's though, but that was to be expected with the power specs. The bass is loose and they lose authority and control from the mids down compared to more powerful amps including the Magni. I know the LCD2's are easier to drive than the HE500's so I'm surprised that the m-stage could drive them to satisfactory levels (and I don't mean volume wise). It's not that I don't believe you hifimanrookie, just saying :)
I've got a 627ap bias thingy coming and so I'll post some impressions when I receive this compared to the popular LM4562.
All in all, the M-stage is great value and performs well above its price, but I wouldn't recommend it for orthos.