Help Building an Audio Room

Pick up a copy of Robert Harley’s Complete Guide to High End Audio. There you will learn why it is so desirable that room dimensions not be multiples of each other. He even shows how to calculate and map room modes. 
Slap echo is a function of unbroken flat parallel walls with little to nothing to do with concrete. You can frame sheet rock over all of it and have the same slap echo. This is handled with diffuser and/or absorption panels. 
The smart move is to build one wall to reduce one dimension to 12 or 13 ft, and use the space behind this wall as a large hidden bass trap. The wall can incorporate storage shelves so it isn’t even wasted space. This wall might even be angled. Mike Lavigne incorporated this exact technique in his exceptionally good acoustic response listening room. 
 

Whatever you do don’t freak out and overreact. My room when it was bare walls and floor was hideous. Merely adding furniture and components is a huge improvement. 
 

The good news is the greatest gains are made from the first small steps. Do those few things, get your system in place and listen. There is no substitute and there is no doing it right the first time. Mike paid big pro’s big money and then still came up with even better sound simply by listening and experimenting. 

Thank you both for the level-headed input.  
I love the idea of a bookshelf/built-in wall to effectively build one wall out and provide some trap and diffusion in doing so. I think that will go a long way, and I can see in the pic they did something similar. 
If I did that on a side wall, I might explore building the front or back wall with a bit of angle; top edge of wall built out 6-10” more into the room than the bottom or vice versa. I’m thinking that may help and be barely noticeable. 
The cost is essentially the same to the builders/me, but as I socialized the idea with them a few weeks ago, they said, “That’s a first. Build it out of square?.. We can do that.” 
I’ll check out the book, too. 
Thank you again. 

Just ordered the book.  Thanks for the recommendation. 

Good job on getting the harley book. Best single source on audio.

I once had a dedicated room in a basement. It was 13, x 15, after framing the front wall and the right wall. Other walls were framed interior to basement.  I had stand mount speakers along the 13' wall about 2.5" from wall with nothing set in between.  Equipment was on a rack to my right. I had DIY acoustic panels behind speakers, at reflection points and behind listening chair which was ~3' from back wall.ceiling was 8.5'. Listening was nearfield. 

It as like having a front row seat in a club with great acoustics. 

I would frame in the three walls so to approach these dimentions. In the Harley book there is a discussion on best room HxWx L ratios. If you frame in the ceiling to reduce height you should consider adding a sound pannel into it at a reflection point. For that matter acoustic panels could be incorporated into the side and back walls as well.  

Good luck on a great project.  

Walls

Even if you do a great job of sound insulating the walls and ceiling, sound travels great through hvac air ducts.  Learned that one the hard way.  Good luck.