Most important fundamentals in your built/modded listening room?

8ft is fine. At 9ft the other dimensions are exact or very close multiples of 9. It is best to avoid dimensions that are the same or multiples of each other. There are always room modes but they are much worse when dimensions are exact multiples. 8x16x24 for example, if you could make it 9 that would be better. In your case making it 9 would make it worse.

If you want to spend money the sky's the limit. If you want to save money for where it will really matter, the single most cost-effective thing you can do by far is use 5/8" instead of 1/2" sheetrock. This costs almost nothing more, but blocks sound an additional 16dB. Walls framed with different studs to inner and outer walls can get you down around -20dB. 

You can go all-out and bring it down to recording studio levels. If you want to spend as much on this one room as the rest of your house. Or you can do the super effective but inexpensive. Highly recommend super cheap. Because what happens, there is a background level when you get quiet enough any tiny little thing you notice- and it doesn't blend into the background any more because there is no background any more. So it stands out. You would never notice the noise coming around the tiny little e gap under your solid core door until the weather-stripping leaves only that tiny little place for noise to come in.

You definitely want the solid core door. With weather-stripping. Either an exterior door, or an acoustic door which will be basically the same only maybe have the seal at the bottom which could be nice. Run one 20A line for the system. Run another for lights and spare outlets.

"...the garage as 18 outlets in it..."

Grow house.

It could become very involved and expensive quick. You say 8 foot ceilings is there anything attached to the joists that can make it less? Wiring, plumbing, HVAC? You can double sheetrock or use something like quiet rock. Decoupling the sheetrock or quietrock from the studs and concrete with resilient channel. Insulate all walls and ceiling. Depends on how much soundproofing you want to do or can afford.

In building, one of the factors that will influence your enjoyment of the room is the sound isolation you build into it -- assuming there is noise in adjacent areas. That was mentioned by others, with possible solutions. I’ll add a +1 to that. People talk about "noise floor" dropping after adding some gizmo that others can’t hear, but every dB of HVAC or traffic noise you keep out of the room is a REAL improvement.

I used solid-core wood doors with weatherstripping, then added seals from this company: https://www.soundproofingcompany.com/

The other side of the coin is that a room that is soundproofed will retain bass, so have strong peaks and nodes. Be prepared to add plenty of bass trapping and spend time positioning speakers and subs. Consider using multiple subwoofers and/or DSP in the deep bass.

Yes, install many outlets, with acoustic putty pads around the boxes so they don’t let noise through. As few wall penetrations as possible. I’d recommend skipping ceiling lighting, and use floor lamps.

Consider what to do about the HVAC. Can you run an oversized vent to minimize whooshing noise? If your system is noisy, would you be better with a "dead vent" not connected to the main system?

If you have a choice, face away from the windows. It is more relaxing. If not, and there is glare, you can have them tinted.

If you live in a damp climate, make sure you are taking adequate care about water vapor (and possibly water) that might seep through the concrete. Basement floors are a specialty. After some trouble, I wound up using needle-punched (synthetic felt) pad with synthetic carpet over it, which lets water vapor pass from the slab into the room air. In wet months, I run a dehumidifier.

Have the door to the room open OUT, in case the best listening position happens to be near the door. Shouldn’t happen with 27 ft length, but still....

I hope you wind up with a great room!

Fantastic set of comments! I am copying these out into a list meant for a contractor. These ideas will be implemented. I hope there are more.

Some replies to you generous folks:

The room won’t have noise issues. Just master bedroom upstairs and my spouse is usually somewhere else in the house. No traffic to worry about.

Thus, regarding noise, my takeaway from the comments is that a modicum of noise reduction is in order, but no need to go crazy. I don’t think I need double walls, etc. Wall damp sounds great.

There are no moisture issues. Colorado is dry.

Electrical: dedicated lines for sure. Two should be enough, no? I have a fairly simple system but there’s a possibility of a home theater system down there, too. Perhaps put the stereo rig on one short wall and the home stereo on the other, with seating toward each in the center? So...three dedicated lines?

Regarding ceiling, this is a case where I might need to do something now and make a further decision later. Here’s why:

• The ceiling with drywall would be about 7’10" with the thicker sheetrock.
• The room is 27 feet long, but it could change when we continue a further basement and house renovation -- so, MC, for now, I suspect that leaving it at 9 feet with insulation and a fabric cover with sheetrock to be added later might make sense.
• Another person wrote to me that I might want to create a false wall at one end (shortening the length to about 25 feet) and have a place to put my electronics. That might help with the problem of "multiples" and also give a neatness to the space.
• I agree about not going crazy expensive.