Room design - home theater

if you're building from scratch here, you might as well do it the right way. If music, you'll do well with the coffered 9' ceiling. I would also NOT do exactly 14' x 21'. You will get much much better sonics if you add 6%-7% difference between multiple dimmensions. (i.e, add 6% to either the 14' dimmension, or 6% to the 21', but not both).
Also, a HUGE benefit of your coffered ceiling is that you can put a massive bass absorber in the ceiling/attic above! In a room of that size - if it's a issolated, sealed in space- you can be assured that you will need to do something with all that excess bass energy. Building a bass trap in the attic space above -accessed from within the inner spaces of the coffering (spaces covered with acoustic transparent fabric- transparent to space above) is an ideal way to balance out the RT time in the room, and make for a supper fast, dynamic, solid bass response from the system. The norm is a slow, fat, overly warm bass sound from sytems, with too much bass overhang covering the sound - especially in smaller acoustic spaces such as this. (yes, 21'x14' is still a small acoustic space).
Regardless, keep the hard dimmensions of the room at multiples that are at least 5%,6%,7% different than each other dimmension, to keep bass modes from piling up and compounding in the room. Results will be a better foundation.
There is of course tons more to the acoustics of your space, building techniques, etc. But I hope this coveres the issues you mentioned.

PVC is an extremely easy material to work with if you want greater choices in wiring used 'in wall' for either power delivery or signal delivery.

otherwise power lines ought to be shielded at least.

I've noticed folks like JPS and others sell upscale electrical and signal wire expressly for use with 'in wall' situations.

There are 'socks' too out there for covering spkr wires and ICs, if needs be which will more closely match up with the flooring colors and decor.

There are also base board stand offs for racing wires behind it.

Another choice for racing wires across the surface of a wall or ceiling is the use of raceways. They can be painted to match the color of the surface they are affixed to and very easy to install…. If things come down to that.

Lending creedance to the above post on overall room size, IF that's cast in stone now, simply adding free standing shelving on the rear wall might help there and add storage for software too, as well as doing triple duty for sound difusion. Too late for me there... my shelving - storage units are built into the rear wall and covered by drapes. Drapes also cover the same span on the opposite end of the room behind the main speakers.

you can hide a lot behind curtains! Stuff you wouldn't necessarily want exposed like sound panels, bass trap panels, a big screen, shelves, etc., and it's a nice look too.

They also difuse the sound if there is sufficient material to develop pleats and of course, the material's own degree of transparency... plus IMHO it makes the room far more natural and home like than recording studio looking.

On a ten foot wide length think at least double and better yet, triple that, for the drapery panels needed to cover it properly. Pleated and hooked are best suited for access behind them, but simple threaded or tabbed panels can do the job very well.

Thanks for the advice about curtains, Blindjim. I have a 100" screen that I'd like to cover when not viewing.

I built a small stage in front of the screen that my LCR amp sits on, so cabling to and from it can be hidden under the stage. For flexibility, small U-shaped channels that can be painted to match walls/baseboards do a nice job of hiding cables, and allow easy access when a change is needed. Cleaning up clutter in a HT/listening room greatly adds to its enjoyment IMO.

I've never seen the term cuffered ceiling, always coffered. I used a coffered ceiling with faux cross beams. The beams hide PVC that carries cables. Ceiling mounted spots controlled by a dimmer provide illumination yet don't wash the screen. The wall around the screen is covered in charcoal colored carpet as is the stage; the ceiling in front of the screen is covered in heavy black velvet.

db

db

Plenty of great ideas there...

it all sounds very nice. I am afraid to cover the screen wall with anything sound absorbent though.

My screen is hinged. It's bottom edge lifts up to expose double louvered bi-folds which open to a storage area in which... at some point... I'll migrate my HT or stereo gear.... once my dedicated ckts get moved.

A clean esthetic goes a long ways to make for a pro looking environment, no doubt.