Stack bags of R32 insulation in the corners and see if you don't mind the dead space ("dead" as in "lacking ambience" -- I'm sure it's anything but "dead" in the low end and lower midrange, hence the bags of R32. 8-12 bags if you can).
The room can certainly be viable and 'dead" -- But the low end is going to be an issue either way - So if you get that under control from the start, you can decide how you want the rest of the room to behave.