Awesome details, Tyson! Thanks so much. I have my blu-tack and cotton and will strongly consider this mod. Your pix are very helpful.
OK, what you will notice with this is that the deep bass will be a bit less, the upper bass will be thighter, the mids more forward, and no echo in the highs. Bass can be boosted by adding more cotton, or putting the grey donut foam back in. However, this will result in a slight attenuation of the highs. Personally I stuffed the hell out of my a900's for the better bass and slightly softer highs (the above pics are not of my a900's, mine have more cotton than that). Anyway, now that you can see how to get in there, you can experiment with foam and cotton stuffing to tune the sound of the a900's to whatever type of sound you most prefer.
Hope this helps!!