I am finding that the excellent imaging of the MA900s makes instruments sound very natural, because the sense of space around the instruments makes them seem more real. So, while the "timbre" (balance of fundamental frequencies and overtones) is not quite as good as the Mad Dogs, the superior imaging makes the sense of realism on the MA900s competitive with the Mad Dogs.
My choice of which to use, would depend more on the difference between lightweight, cool/airy and "leaky" versus heavier, hot/warm and well isolated, rather than any audio differences. Even though there are many audio differences, the different strengths are roughly equal.
In contrast, the HiFiMan HE-400 lose out in a comparison with either the Mad Dogs or the MA900s. The imaging and soundstage on the HE-400s are nowheres as good as the MA900s, and the timbre and midrange clarity of the Mad Dogs are superior to the HE-400s. The HE-400s are the heaviest of the three, and have no isolation whatsoever. So, while the HE-400s are very enjoyable headphones, they lose out to the Mad Dogs and MA900s for my use, at this time.
I would agree with most all of this. The MD and ma900 would seem to make an excellent pair of complementary hp's on a budget.
I didn't think anything could be more comfy than my hd650, until I got the ma900.