I would choose the O2 over the M-Stage.
The latter is warm and, IMO, you don't want that with the somewhat dark sounding Mad Dogs.
With the Fostex HP-A3, I use the following EQ:
Tuning down the 300Hz region makes all the difference to my ears. It opens up the scene and I gain a lot of clarity. The slow treble bump is there as a compensation to the change in the 300Hz region.
With this EQing, the Mad Dogs directly competes with the HE-500.
They are less technical, but more balanced and the joy is somewhat the same between the 2 (which says a lot!).
To get similar result on the Cowon J3 + M-stage combo I tune the 270Hz region down to -3 (I don't know the scale, certainly not in dB). I also compensate a bit in the treble but (proportionally) less so than on the curve above.
Edited by Clemmaster - 2/7/13 at 12:32am