Advice on dedicated room

We are all agreed you need to make the room more oblong.

Why not build a separate storeroom on one side for records, books, equipment, whatever.
Build about 5-6 feet wide reducing the 16.5ft dimension.
Clear all the crap out of your room.
I did this when I built my room.  I have art on the walls in place of loads of LP racks and a great open feel.

More than anything else it helps just to keep in mind. No room is perfect, and one can easily spend more on the room than the system.   

Back when I was searching around going to all the audio stores within a couple hundred miles the best sounding system by far was in a room just like yours. Corner Audio, only the long axis was vertical. The room was roughly 15x15 with a ceiling twice as high! Yet he had easily the best imaging, most engaging sound of any store up and down I5 from Seattle to Portland.   

That is the main point I would like to make. Harley has an excellent book, and there is a tremendous amount of information. These things are all about compromise. The more you know the more likely you will be happy with your particular set of compromises. 

I’ll tell ya what works for me:

My room is 19 x 29. The ceiling has a gable on one side and dormers on the other. The gable roof is 12 ft at the apex, 3 ft at either wall.

The ceiling is covered with 17” of rock wool. The walls have 6”. NO SHEET ROCK: I covered the insulation with fire resistant burlap. 
My 3,000 book snd 6,000 LP library line some of the walls. 

The floors are plank hardwood. I cover the floors almost completely with throw rugs. Only four small windows: blankets cover the windows in the dormers during listening.

This has given me a fairly dead, semi-anechoic room. The acoustics are superb. Most people notice immediately upon entering the room the difference in how their voice sounds.

The room was added to my system three years ago. My system has otherwise been the same for 36 years. The room lifted the overall quality to a place where I don’t feel any need to be looking for a anything else to make it better. I sit back and enjoy the music.

My room was designed by a Cooper Union/Harvard trained architect (and friend) who came up with this concept with a classmate who is a “sound scientist/architect”.

The Square shape of your room is a problem. I would put a wall in to create a 4.5 ft wide room along the length, and put your equipment in there, and your record library in there (yes, I’ve made an assumption there, but whatever). 

@ronboco, the concrete walls are not bad as a surface inherently.
Concrete is damped / well damped ( below grade ) but very reflective.
The entire audible spectrum will be reflected making diffusion and absorption more important.
Addressing the low end with the ISO wall is a very good idea and find out how much benefit can be expected. If drywall is put up then damping the resonant areas behind the finished wall would be important.
The concrete walls help with sound isolation as well.