I had a nice conversation with the manufacturer of my speakers yesterday afternoon. I wanted to ask them what helps make their particular speakers perform well. A couple of things came out of the conversation, some of which corroborates the information I've gotten here. Without any prompting from me, he agreed with the MDF/drywall sandwich with the GG in between. In fact, he went on to say that any two dissimilar materials would work better than two layers of the same material. Around here, OSB is cheaper than plywood which is cheaper than MDF. Interestingly at HD, MDF looks like it only comes in 2' x 4' boards and only 1/2" and 3/4" thicknesses. So it seems that a layer of OSB and then the two layers of drywall would be most cost-effective.
He also supported the idea of isolating the speaker box (which should have an internal volume of 2.25 cu. ft.) from the framing of the speaker wall. He has used rubber pads but liked my idea of IB3 clips.
I had a hard time explaining my open at the top and bottom design of the speaker wall with the rationale of using the space behind the speaker wall as a bass trap. But he kept mentioning that there would be a rise in output of some frequency at any falloff of the otherwise flat plane of the speaker wall drywall dependent on the distance of the dropoff from the drivers.
He also threw in a tidbit about people wanting to use speakers without their grilles. He said that they measure the output of their speakers with the grilles and then tune out somehow the expected peak (he said in their speakers it ended up around 3000Hz,). He went on to say that running the speakers without the grilles would then be expected to display a drop in output at the same 3000Hz.
At the end of the day, however, he suggested building the room with all the little tweaks you want to include that follow generally accepted common wisdom. But that you then have to actually measure the room to get the finer details right. He said that with careful, thoughtful initial construction, the fine correction could be achieved either electronically with DSPs or with acoustic treatments throughout the room.