Building a house

I agree that you need to think about sockets and wiring in principle. From experience I can say that it is very difficult and expensive to redo something.

@dodgealum  Just saw your post. Hope this is not too late.

I did an old world solution for my home theatre: sand. I put mixed fine and coarse sand into the spaces between joists, about 2".

That is: at the framing stage, double or triple all joists - I used LVL, screwed and glued. Screw and glue (elastomeric M1 works well) a layer of plywood to the bottom of the joists. Fill the cavities with 2-4" of mixed sand. Get an engineer to sanction this.

What you have is a preloaded floor, immune from squeaking and fairly soundproof. I guess you could add rock wool insulation above the sand for the last added iota of sound proofing.

Worked for me. YMMV.

I stumbled upon this thread and thought I'd share my thoughts. I love the idea of a dedicated listening room, and your plan for the walls sounds great. For the floor, have you consider adding an additional layer of plywood with Green Glue sandwiched between them? That should help with any potential shudder.
On a side note, I'm also looking to upgrade my bonus room, and I'm thinking of adding some French doors for more natural light. I discovered doorsdirect2u.co.uk and was wondering if anyone has used them before and what their experience was like. Any feedback would be appreciated!

I understand that Green Glue is not a glue - it is a sealant or filler. I prefer something that contributes both actively and passively, like M1. Being a strong adhesive, M1 bonds the two layers together, adding their strength. This benefits the occupant of the sound room by rigidifying the walls.

Being elastomeric, M1 also dampens. This benefits those on the other side of the walls who aren't forced to listen - but not as much as Green Glue, I suspect.

Being selfish, I went with M1 and am well satisfied.