Nice written review and comparison I also like your video reviews pleasant voice well articulated and informative covering the major points about the headphone sound,build, and comfort. From the sounds of things I would prefer the AD900X over the MA900 being brighter more lively headphone and having the larger sense of space to the sound despite not having as precise imaging. As long as the sub-bass is a good improvement over the AD700 in extension and quantity at stock I'll be happy I should be able to get them where I want with the Hardware EQ on my soundcard and still have a good amount of volume left. Also I intend to use them 100% at home at my computer where I'll mostly be listening to music using RPG Wizards Laid back Dolby Headphone config in foobar2000 which will further increase the sense of space and expand the soundstage to be more outside you head and improve imaging as well.
Thanks for your comments :) It definitely has an improvement in sub-bass and a general improvement over the AD700, and the AD900 as well (from memory). Glad I could help you make a choice :)
It sounds like you like a nice wide soundstage and out of head sound. Why not just get speakers? Or K1000's! Hehe.