From the walls outward we had concrete, then furring strips with rockwool in between, then Kinetics Isomax, then hat channel, then single layer 5/8" damped drywall. The shell acts as a "floppy wall" to provide damped low frequency absorption. Ceiling construction is the same. The room dimensions were optimized to prevent modal stacking.
Acoustic treatment wise Len moved his RPG BAD ceiling cloud in from the previous room. I believe we added some additional absorption above it...Len can confirm.
Then we took a break whilst deliberating the best approach to finish out the room. We already had a concept involving floor-to-ceiling columns all around the room but we decided to change it to reduce construction complexity.
I like to use a "light" touch with respect to acoustic treatment, and to try and keep the room as live as possible.
We added 6ft tall Listen Audio diffuser columns. These are an interesting product, offering some absorption and some diffusion. These cover the opposite sidewall reflection points and prevent left/right flutter echos across the seating area. The immediate lateral reflection points are treated with RPG BAD ARC, which is a curved version of the BAD.
The wall behind the speakers has four Modex Plates. Two are broadband (the center two) and the other two are Type 1. I specified some broadband to provide absorption of any diffracted sound wrapping round speaker baffle and also to manage the mid/high frequency decay times in the room.
The wall behind the listener is a custom made box fronted by RPG BAD panels with a 10" pink fluffy insulation fill.
About the only thing left to do is to cancel the longest axial length mode with a subwoofer or two.