How To Proceed With Room Treatments

@milpai I am considering adding tri-traps as well in the back corner.  What height did you go with? Floor to ceiling or just partially?  I have the wall treated with 24x48-inch panels behind each speaker but my corners aren't treated in the rear and I know I'd benefit from addressing them.  Aesthetically I really need to keep a partial height - no more than 48 inches probably.  Just curious what's working for you.  My room dimensions are similar - 15x20.

@three_easy_payments,
I went with entire height, floor to ceiling. But the back wall is slightly lower. So I have 48" + 45" tri-traps, leaving 1" to the ceiling. The bass tightened up considerably. You can feel it in the chest. Treble clarity and separation also improved drastically. 

Not everything has to be a trap. You want a mix of absorbing as well as diffusion. Every room has areas where bass is a problem (corners for example). Then you have 1st reflection points and this could be the sidewalls, ceiling and floor.
GIK and ASC are excellent companies or deal with Acoustic Fields for designing and building your own diffusers

jrpnde asks:

is there any sort of experiments that can lead to a final room treatment that can optimize results before buying the rather expensive panels for that purpose?

Yes. What you want is Owens Corning 703 https://www.amazon.com/dp/B006FY7RHY/ref=dp_cerb_2 This is 2" but you can use 1", get one sheet of each if you want to test which you like best. Only slight difference as thicker absorbs a bit lower frequency.

OC703 is developed specifically for acoustic absorption and is used in a lot of expensive panels. It cuts easily with a razor blade, sheet rock knife or table saw. By itself, before covering, it is so light you can attach a whole panel to the wall with only some stick pins that make such tiny holes you’ll never notice, perfect for testing.

The most effective location to test first is to put 12" triangles in the upper ceiling/wall corners. Next is 8 to 12" wide by 3’ or longer strips vertically in the corners where the walls meet. Third place usually will be two pieces about 12" to 18" placed on the side walls at ear level to catch the first side reflection off each speaker.

Most people treat side walls with a lot of material to kill slap or flutter echo. The room is easily over damped, especially if it already has carpeting. This way uses a lot less material by treating only the most effective locations. Corners matter greatly because any sound emanating from a corner is reinforced by the walls. Damping in the corners is therefore much more effective than the same area of material on a side wall. Except the reflection points, where it has a bigger effect.

This is all stuff you can test and confirm in an afternoon and for cheap. Just like you asked.

GIK will ship you Owens Corning 703 in a box of six for about $100. 
The panels are 24"x 48" and 2" thick. Enough to play around.
The cost to benefit ratio is ridiculous with their finished panels. Can't go wrong.