So, I discovered a small, easy mod for the MA900s.
I was listening to something with significant bass content, and I was adjusting the position of the headphones. I happened to rest my hands lightly on the sides of the headband and adjusters, and noticed that the bass became a little firmer and less wooly.
After some experimentation, I determined that this mainly occurs due to looseness at the point where the sliders enter the headband. I found I could get the same improvement by lightly grabbing that point.
So, I took some "Gorilla tape", which seemed ideal because it is a ) black, and b ) has very strong adhesive. After getting just the right position of the sliders, I wrapped one thickness of Gorilla tape around the end of the headband and around the slider. I also wrapped some around the pivot point for the earcups, because the same principle should apply - helping the headband and yoke to be stiff, and thus provide a more solid and stable anchor for the drivers to move. (Remember that speakers and drivers are motors that push air.)
After I did this, resting my hands on the sides of the headband no longer changed the sound, proving that it works.
Note that this is a small sound change - do not expect any huge improvement.
But it is easy to do and easy to remove.