After 18 months of design work and a major relocation within the design envelope my new home and dedicated room are under construction. I did a very deep dive on size, materials, construction techniques, electrical service, the works. What I learned is that there is no end to the lengths you can go to design the "perfect" room and there is a corresponding limitless amount of money you can spend.
One thing I can share is be sure to identify your goals. What are you after? I had three goals in mind:
1. Make the room sound good without expensive treatments and without the room looking like an audiophile man cave. I wanted the room to be pleasing to the eye and look like any normal room in a house. (Albeit one with a kick ass stereo system and a bunch of record albums);
2. I did not want the sound to migrate from the room. I want to be able to listen at volume and not disturb my family--any time of day or night;
3. I did not want noise from the house, including from the mechanical systems, entering the room.
The room was relocated from over the garage to the basement. That was huge. Instead of a suspended floor and wood framing/sheet rock, now I have concrete floors and 2 walls with sheet rock over steel studs (Superior Wall foundation). I was able to land the golden ratio and 9'6" ceilings.
Ultimately, I only went so far with the wall construction: Staggered studs on walls common with the interior of the house, Rockwool insulation in the cavities on all four walls, two layers of drywall (1/2' and 5/8") with green glue between, walls and ceiling. Entry door with acoustical seal, acoustic putty around all the outlets and switches.
Multiple dedicated lines on the same phase, high quality but not insane outlets (Pangea Premier Series). Siemens panel with copper buses, copper coated ground stake, orange 10 guage wire to the dedicated lines.
I am working with the HVAC guy to minimize noise from the system and have all the mechanicals isolated in a separate sound mitigating room.
I could have done a lot more with unlimited funds but was determined to approach this with an eye toward diminishing returns and to not overdamp the room or have it look uninviting. I'll be running Sound Lab stats so reflections are less an issue for the sidewall, floor and ceiling. I'll lightly damp behind the speakers and diffuse on the opposite wall and then experiment from there.
Fingers crossed I made good choices. I hope this helps you to do so as well!