Most important fundamentals in your built/modded listening room?

So one thing I just discovered is the joy of diffusion.

I've heard well balanced rooms before, with a mix, but only recently have I been able to do A/B comparisons with and without them. 

If anything, incorporate diffusion into your planning as soon as you can.  They add air and shimmer without glare. 

@erik_squires
I agree about diffusion. I’ve now gone the over-damped route and am seeing how to make my space "alive" without brightness. So...

I have two of these: https://images.app.goo.gl/rkE6t1jUgAMBoHCm7

QUESTION: How would you lay the GROUNDWORK for diffusion?

@mesch

Thanks for your reply. Some questions below. I will design the space for audio, once the kids leave. I’d probably just have a different amp for video, as I don’t want to run tubes for video all the time. I already own a Denon AVR and klipsch speakers for A/V. Maybe I’d ditch the speakers or put them on surround duty.

Insulate between joists with two layers of 6" insulation covered by 5/8’ drywall.

QUESTION: What kind of insulation? R38?

The 17’ dimension can be played with by furring out the concrete walls as desired. Insulate between wall material and concrete. Use 5/8" drywall for wall. Consider this the width. Frame in a cross wall to the desired length.

Given you are running Mono amps and a pre with digital source I would route 3 dedicated lines to the room for equipment, one to amps, one to pre, and one for digital front end.

QUESTION: If I had a separate set of components for A/V would you do 4 dedicated lines? That seems like a lot.

I would consider placing a equipment rack for sources and pre at one side of listening chair. Mono amps on stands where best suits.
Have nothing between speakers.

QUESTION: Makes it hard to have a TV, but I suppose if there’s a "false wall" it could hide the TV. Sound ok?

Room ratio is a tough one. If one looks at various ratios, some say 1 H x 1.67 W x 2.7 L (P.S. Audio) and that works out, with an 8 foot ceiling to 21.6 x 13.3 x 8. That amounts to reducing width by almost 4 feet and the length by 6 feet. Not sure how that will work for various purposes. This is why I was thinking to keep the ceiling open (without drywall) and have it at 9 feet. That way I could have a ratio of 9 x 15 x 24 as a target and just need to pull off the walls by a couple feet. Still, I guess once you start building out from concrete (one exterior wall) you can probably bump out pretty easily, eh?

I had multiple goals.  I have a 50/100 Hz resonance in the room that needed treatment so I added GIK's 7" thick bass traps with diffusion panels on top. You don't have to go that thick, unless you lack where to put bass traps, but the GIK Impression panels are very effective and very affordable compared to the prices I have seen on some QRD types.

When we finished our family basement in the early 1960's my father did not enclose the ceiling in order to avoid additional property taxes on "liveable space".

In Des Moines, @ the time, this was how it worked.

We instead sprayed the ceiling and ductwork with flat black paint and the open ceiling was not very noticeable.

DeKay

@hide45  Regarding the ceiling, I agree one could leave it open. Might consider incorporating bass traps where ceiling meets walls. Maybe insulate some sections and cover with fabric as per MCs suggestion. Also spray a thin layer of closed cell foam insulation to round out edges where joists meet subfloor above. Basically having diffusion and some absorption installed into ceiling. Allowing for the 9' ratio reference.

I would mount TV flush to wall behind speakers. Maybe elevate it above speaker height. Cover with absorption products when not in use. Might frame it in for this purpose. 

If video electronics are not in use when music system is used then one dedicated line could be used for both. In your case the position of the dedicated outlets will need to be well thought out regarding topology of A/V systems. 

Just some thoughts, I am enjoying thinking on your project.