A fantastic comparative review, well written and layed out as usual. An the vids are very finely cut as well.
I've had the AD900 in the past, I certainly feel the MA900 is technically better in most respects, number one would be the bass response (duh) but no less improtant is the improved(!) sense of oppeness and the out-of-head coherent imaging. AD900 had "bigger" sound but less defined. I'd never decribe AD900's treble as agressive tho, Do you think the AD900x is brighter?
One thing I don't agree with is your assesment of the build quality and design:
"The MA900 is quite literally one of the most dismally uninspiring builds I’ve ever seen on a headphone. The super light weight, the flimsy way the earcups flop around in their cradles, the all plastic design (apparently there is actually some metal hiding the headband), the ill-proportion of the massive driver size to the tiny headband. The cable is thin but the plug is okay. Almost everything about it seems flimsy and cheap."
Well first, While i don't own the MA900 anymore (I liked them but i've became a spoiled SQ brat:)) i was absolutely postive that the arms and yokes are magnesium, not plastic, As the driver frame itself clearly is. It's just a differnt material and was colder to the touch during winter. Possbly the outer grilles as well, Not sure about that. You sould check that again, Maybe put them in the fridge for a coupla mins. :)
Second, You've ascertained yourself that one of the design goals was to minimize the overall frame in favor of wearing comfort. Do you think there's a better approach to achieve this that to err... minimizng the frame? In comparison to the F1, Tho objectively i think it's still comfier than MA900-due the the supperior weight distribution of the self adjusting headband- i did feel the MA900 was somhow less intrusive on the head because it's so slim... I dunno, it's somehow psychologically made me less aware i'm having them on.