For sealing the back of the cavity - a single gang deep box, surface mounted, with the cable opening caulked closed to the box - really helps. The just install a single gang tech plate with an NL4.
Studs for the cavity walls/ceiling - forgot to note, typically I call out 8" on center, to keep the amount of sheetrock flex to a minimum.
It doesn't hurt to put some acoustical absorption between the entire stage joists to soak up whatever sound is under there. Mineral wool batts work great and aren't expensive, aim for the 7.5" thick ones, and put them so they're towards the bottom of the joists so they have air above them. This isn't due to subwoofers per se, just to mop up whatever remaining gak is bouncing around under there.
A layer of some sort of decoupling between the floor layers, like felt, also helps keep the stage surface from transmitting sound like a giant drum head.
The top of the sub cavity can be used as a load bearing surface, if there isn't space for a cavity top, gap, and then joists/floor. Between the 2x4's that are8" OC - also use standard P1000 type Unistrut and washers/shims between the studs since they are load bearing. The studs allow the sheetrock underneath to be screwed to it, while the strut supports the weight of the stage.
The under-stage area and the audience area should be separate (sealed).
Something else I've done if you want it extra decoupled - add a retaining wall around the cavity, and fill it with sand bags, with the bags touching the cavity walls/ceiling. That's overkill but works really well!