Diffuser or absorber behind listening position....and is it worth doing??

Change the wood blinds to heavy drapes and you do not mention the floor but that can have a detrimental effect on the sound especially if it is hardwood, ceramic, or some other hard surface. If it is a hard surface floor try a heavy area rug in front of the speakers. With just these small tweaks you will have a much better sounding room.

I have been at this in my room for 7 months. I started off just trying things here and there and listening. The number of places to put things seemed infinite. I had blankets and towels everywhere, bought some panels, moved them around. It was time consuming and crazy making. Then I took some advice I had been resisting -- buy a $100 mic and download REW, Room EQ Wizard. Learn the basics. Measure, treat, listen. Repeat. Learn where your room’s reflection points, peaks, and nulls are -- for real. In some cases, theoretical predictions were correct, in other cases, they were way off -- because I have an odd room.

I bought OC 703 panels and wrapped them in fabric. With 6 of them I made two 2" panels and 2 4" panels (doubled up in side a fabric pillowcase). These proved to be very effective tools to test out various effects in the room, especially when doubling them up and leaving space between them and a wall. They were excellent preliminaries -- in conjunction with REW and listening -- and gave me confidence about how much treatment to invest in later. Throwing money at the room acoustic problem is more wasteful than throwing it at gear because it's much harder to recoup by selling. And it's hard to integrate into an existing domestic arrangement.

Set some goals, get the right tools, work patiently and incrementally. My two cents.

Hilde45 - I have read several of your posts regarding your journey to manipulate your room's acoustics and they were a good source of motivation. Our reasoning and approach to improving acoustics seem to run parallel, including cost and resistance to use EQ technology (for now anyway) . Although I believe I have the added challenge of storage and convenient repeatable implementation.....uhhhhg:) 

Anyway I pulled the trigger on 12 panels of 703 and other materials, it's about $275 all in. So If it doesn't work....well, it's wouldn't be a big financial hit. Plus I have a consumer HT system in another room that SQ would definitely benefit from, and it would be easier to implemented without the added hurdle of portability and aesthetic restraints.

Speakermaster; 53B - unfortunately the addition of rugs and drapes aren't an option. But a quilted throw is an option.Thanks

By the way, if all goes well I plan to post images of room with panels implemented vs stored in "my virtual room". 

Thanks again

The advice to acquire a mic and download REW or HolmImpulse, as used by Earl Geddes and also free, is solid. Playing with this allows you to see the results of added treatment.

The basic problem all rooms have is the sound taking too long to decay and the use of EQ simply can not reduce the decay time, it needs to be absorbed and the OC703 you bought is great for the job.

tomic601 mentioned RT60. This is the time it takes for the sound bouncing chaotically around your room to decay by 60dB. and without looking up the figure for your size room would, I guess, be about 400ms. So if you see some frequencies with a T60 of 1 sec. these are the ones to be tamed. Then after introducing and/or moving some more treatment take another shot until you achieve an even smooth response across the full spectrum. Great fun.

By doing it this way you avoid any guesswork and will come to realise that the suggestions to hang a drape or install a thicker carpet is like trying to win the jackpot.

The thicker the absorber panel the greater the range of absorption. So 6" will be more broad-band than 4"  Move away from the back wall and use absorbers. You would be too close to a diffuser on the back wall.

Have a good read of this: https://ethanwiner.com/acoustics.html#bass%20traps

@mrpsync  I'm glad my diary of attempts was helpful. I really fumbled a lot in the beginning, but earnestly. You did well buying 12 panels, by the way. That gives you sufficient quantity to actually do some very real experiments, especially, as Lemonhaze points out, using 6" panels. You can really affect bass peaks with that thickness if it's also some distance from the wall.

The other thing I'd add is that you should not get too hung up on building frames for these panels; fabric that is adequately transparent (burlap, muslin, etc.) will protect you and enable you to move things around. 

One other helpful hint I got early on was that if you're trying to troubleshoot a ceiling first reflection point and it's a pain to get a pad up there, try one on a spot below on the floor. That should help show some difference without all the ladders, staple guns, and cursing.