Thanks JPA and IUsteve, I'll search around for some other threads on filling stages and/or risers with insulation. I can understand how a riser could act as a sound deadener, since it is in front of the sound source, I'm not so sure my idea for using the stage is so good as most of the sound is originating from the stage, although the rears and surrounds are more or less pointed at the stage, so maybe it is worth it. I'm thinking by adding some Roxul inside the stage cavaties, I could at least reduce any boom box effect.
Anyway, the soffits are almost complete, ran out of 2x3 lumber tonight so I had to stop short of finishing the rear soffit. I'm planning on drywall from the front/outside lip to about 1/2 way back (12"), then some sort of fabric the remaining 12" (24" total soffit depth), this will allow the Roxul to absorb some of the reverb in the room.
Spent about 3 hours this morning re-locating the HVAC supply boot, I guess I didn't think ahead too well, where I had it placed it landed just in line with the 2x3 upper stringer for the soffit, so it had to be moved. Keep in mind the joist cavities are packed with cellulose and covered with 2 layers of drywall and GG, so it was a real PITA, but it's finally done and will end up exiting underneath the soffit.
Got some of the Roxul installed, seems to make a difference immediately with regards to the echo/reverb in the room.
That's about it for tonight. The wife is about tired of hearing the sounds of miter saws and drills for today, but she will like the end result I'm sure!