Treating Floor in New Construction - Reducing Footfall and Vibration

I suggest you check the 'Acoustiguard' website. I envy your opportunity to create a glorious sound room of about the size I would appreciate.

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

I would glue wherever plywood touches wood ; okay a bit o/c, but results are impeccable. Cheers. Peter

Do you have questions re your wall and ceiling construction? Vital as well. Don't skimp. Big room,big sound,big problems : )  Pete

For decorating reasons I would go with all parallel walls. 10 foot or higher ceilings provide more flexibility for multiple levels,middle area treatment and massive crown mouldings. As a listening room I assume some sound treatments will be used for standing waves,etc. I live in the Vancouver area in Canada where it is not uncommon to install hot water radiant heating in concrete on all floors in better homes."Double" 5/8 plywood subfloor is minimum where a tile floor will be over the concrete in-floor heating of minimum 4 inches of concrete. Now the floor is stiff! and massive!! Enjoy 

Instead of 2 x 5/8 drywall I experimented with 1 x 5/8 MDF under 1 x 5/8 drywall and was amazed at the improvement overall.  I used adhesive on the studs, probably PL400,can't remember. That's how I would go now,all around.  Seemed to put the sound on steroids for clarity,dynamics,nuance and overall sense of aliveness.The room was incredibly "quiet". What about your ceiling?

Also,forgot to mention. Re the floor; 3/8 plywood glued and screwed to the underside of the joists gives 'tremendous' boost to rigidity and is how I would go if building new or renovating today. Between floors I like a second layer of press fit insulation resting on the plywood base.
I'm with needlefree re the benefits of QuietRock drywall. They claim 1 sheet of their 5/8 is equal 7 sheets of normal drywall if I remember correctly.