Advice on dedicated room

Sound insulation behind a drywall will essentially do nothing for the room acoustics. It will make elsewhere in the house quieter. If that is your aim then good. Double thickness drywall will help a little inside the room, ensure the two surfaces are very well glued together and very well attached to studs. I have smaller room than this with 45 degree corners, that seems to help a little. Remember sound doesn't know what size the room is it is purely a mechanical acoustic response to the whole environment i.e. furniture, carpets, surfaces (hard / soft), angles etc. There are three aspects reflection, absorption and diffusion. You usually have a mix but rarely the right mix. Carpet the floor maybe plus rugs, the ceiling is a big flat space, and reflective but quite a way away. Some diffusion is good, two ceiling fans perhaps? I have a very large recliner in soft material and its effect is dramatic. Other softer material seats help and I have built some prototype bass traps that diffuse, absorb and use membranes, they do work but are difficult to get right. 

room modes online calculators abound…note that they don’t include a non parallel wall…hint, frame that in now. Also you want low noise in the room, so insulation will improve the sound. S/n is important. 

Jim Smiths book is great - Getting Better Sound.

Fix your room before adding subs. It’s not all about bass. 1st reflection points have nothing to do about bass. Talk to experts from GIK/ASC/Acoustic Fields to get your room right with diffusion and absorption, then start adding whatever you think you need to make your system sound the way you want

I spent a gob of money on a dedicated two-channel room 2 years ago, managing to avoid most of the usual pitfalls.

One of the most successful money pits was Quietrock 545. It REALLY works. 5 layers of drywall, one layer of sheet steel, 1 1/4" thick. Thing about walls is that they flex with the sonic compression wave, turning your drywall into a very low class speaker. Q 545 is a beast that does not flex. Good sound insulation to boot.

Glue and screw construction. None of this nail gun crap. I used two elastomeric products from Chemlink, M1 and BuildSecure. Many cases.

Dimensions also matter, as your intuition tells you. Don’t listen to the usual suspects and their snake oil here; actual scientific research has been done on this subject, and it’s absolutely free. Check out the School of Acoustics at the University of Salford (UK). They did a quarter million simulations to find that MOST rooms are poor, a third are OK, and 2% are good. Half inches matter - I’m serious.

Good luck!

Some diffusion is good, two ceiling fans perhaps?

The air transmits your high fidelity sound. Setting it in motion with fans cannot be a good idea.