Interesting question where my experience may help.
You mentioned footfall and vibration.(Flex is also important)
Mass and rigidity are the factors.
Your decision must consider the "plywood" thickness used
throughout the adjoining areas;and the depth of finish flooring in
those areas.(There's nothing worse than irregular flooring-and I've seen it in luxury penthouses : ( ...
--The heaviest thickest plywood is best;glued and screwed (don't consider even 'ring' nails) " carefully" installed (i.e. supervised,we do not want a any rushing on this)
--I prefer 3/4 inch "Marine Grade" -you're research (check out Homestead Marine) will tell you why-- 'T&G (tongue and groove) plywood with high quality adhesive on every inch of every stud -- that retains some flexibility.
Check out PL400,PL Premium, Titebond,Sika & 3M products with your builder/carpenter. I like bldr to be involved in the choice in any excellent product they like to use;they're pro's and 'pro's have preferences'.
I like your carpet (over heavy rubber underlay) with wood floor at the speaker area;it would be my choice (my speakers weigh >450+ lbs each).
--Regarding the framing or joist spacing I would be comfortable with 16 inch spacing - with carefully installed "blocking" between the joists. I will be using 'tight' fitting insulation batts of either used cotton or Roxul Rock wool soundproofing type - carefully installed right up to the floor above (I would go to the extreme of using spray glue for permanent adherence to the floor).
Your carpenter should fasten (screw) at 4" along the joints and perimeter and 8 inches elsewhere.
I would have all joints glued as well to eliminate 'voids'.
Note re joists. 2x thickness gives double strength and rigidity
"depth" " " " & 4 x rigidity.....
Gotta go but welcome your questions.
Pete
