How to tell if Acoustic Treatment is Needed?

Definitely try alternative placement ideas before you spend $$$ on the problem. If nothing else, it will help optomize what you already have.

Then get some coush cushions or quilts and oth4er dampening things (borrow some from other audiophiles if you can) and try them around the room. In the corners as bass traps and at the 1st reflection points. If you are like me, they will be revelatory, and then there's no going back.

IMO treating a room is almost always necessary and rarely does harm unless it is overdone (or done improperly).

Bob

Shadorne is 100% correct. You should move your listening chair away from the back wall. Try getting about 6 feet away from the speakers. So your ears might be 3 feet from the back wall and the front of the speakers 3 feet from that wall. Give that a try and let us know. It is cheaper than room acoustic treatments I believe.

If you move six feet from your speakers you will also, very likely have to bring the speakers together as well in order to keep the soundstage focused. This, in turn, may also reduce your soundstage and imaging abilities to a smaller space. This is based upon my own experience with various speakers in my own space. I can tell you that in my room of similar proportions, that arrangement also looks pretty ridiculous and closes the room off to a significant degree. You mentioned you have a coffee table which makes this proposition even more challenging. It rates extremely low in the WAF. Hell, I don't even like it, in every way imaginable (visually and acoustically). Diffusion on the back wall is a far more pleasing solution for me. BTW, speaking of the coffee table - you did mention that you'd tried moving it away from the chair. You might try two other things: eliminate the table all together and see what that sounds like. The second is to leave the table in the same place you use it and find the actual reflection points on the table. To do this place a mirror flat on the table surface. Sit in your listening position and move the mirror around the table until you can see the reflection of each speaker in the mirror (two separate spots on the table). As a test, in those spots place a pillow or a small plant or something that my otherwise absorb or diffuse the sound. See if that makes a difference for you.

Keep in mind, that none of this stuff may occur to you to make a difference at all, or perhaps it makes a difference, but one you'd really have to make an effort to hear yourself. There's no doubt the physical changes in the room do make a difference, and that treating a room can have a positive effect. The question really is do you hear it, is it worth it to you, and do you care about the differences it does make. If the answer is no after giving it a try, then keep on enjoying what you have!

i agree that long wall speaker positioning is the way to go (as it greatly ameliorates side wall reflections), but it does create its own problems:
1) bass articulation
2) staging
3) top end extension

IME you cannot overload the room w/ bass traps---put them in the corners to start (lots of DIY solutions for very little money--search Jon Risch). but you've also got to reduce standing waves between the walls that are 12' apart (check for slap echo; if its present, add wall panels--OC 703 is a nice product, put that covered w/ some polyester battling over top on the wall behind your head and see if you notice anything. its a $50 experiment).

Thank you for all the wonderful suggestions and links. With the speakers on the long wall I can't move my seating position or I'll be sitting on top of them, but it is something I will keep in mind for the future should I change the speakers' position. At present I think I'll try some diffusion on the wall behind my head.

What I think you'll need is, absorption on the wall behind your head.