Studio Acoustic Tiles/Panels for listening room?

Found even a little sonex or acoustic tile does something bad to the sound, maybe changes the phase, who knows...

Bufus is right. The attenuation is in the middle band and upper frequencies. It is likely to through the RT-60 (reverberation time for 60 dB attenuation) off. You typically need a variety of absorbers and diffusors to get the right balance in a room--I've never seen one material work.

Nonetheless, a little Sonex or RPG at the first order reflection points does wonders for signal to noise and coherence. I am now using the RPG "Studio in a Box" which inlcudes thick, wedge shaped pieces for the corners, large panels for absorption, and smaller tiles which cna be used in a grid pattern on the wall to create a diffusive effect. Combined with 3 RPG Skylines behind the listening position and with Corner Tunes, it works wonders on my otherwise far too reverberant room. Not someting, of course, you'd do in your living room unless you are single or looking for a good excuse to divorce.

Alexc hits a good point. If you don't have absorptive material at your first reflection points, you are smearing your image pretty bad. This is not a big problem if you have your speakers on a long wall. I had mine set up on a 20 foot wall and my lateral imaging was good. When I went to the 14 foot wall in the same room, I had problems. Sound absorbers, drapes, etc at the first reflections points, really made a big difference in this case.

To find your first reflection points, hold a flashlight or laser pointer at the top of your speaker and shine it on the side walls while someone else holds a mirror flat against the wall. Adjust the flashlight and mirror until the reflection hits your listening position where your head would be. Now, mark the spot where the mirror is at. Do this for both speakers. Then do an experiment of listening to some very familiar music without any absorption and then again with some type of blankets, absorbers, drapes, etc. You should be amazed at the difference.

The room where my rig sits is an acoustic disaster. 14 feet long (speakers are here), 11 feet wide (where I sit to listen), and 8 feet high. The right wall is mostly a giant sliding glass picture window. The right has a passageway which goes on 5 feet to open up to the sink and vanity mirror, and then further on another 5 feet is the toilet.
The left wall ends up in a door, which I keep closed when I listen to music. Essentially, the room is an inverted "L" when the door is closed, and an inverted "U" when the door is open.

Will getting a set of RoomTunes help here?
Note that I cannot move anything around substantially because of living constraints. The speakers are Mission Freedom 753 and are 2 feet from the wall, 11 inches from the rear wall (Mission recommends at least 9 inches). I was thinking of getting two lengths of Iranian carpet, 3 feet wide and 7 feet long, and hanging them behind each speaker, in addition to the RoomTunes.

When I do the "clap-test" I do hear an echo.
Basically, I have a huge mid-bass hump in the room.

I have 2 framed art prints mounted (no glass) on wood on the left wall, which I kind of converted into quasi-panels by spot-gluing cotton balls in the hollow area of the rear of the frame. The 1st panel is 3 feet wide by 4.5 feet long, and the 2nd is 3 feet wide, by 2 feet long. Both frames are rigidly mounted at a distance of 3 inches from the wall.

Thank you for your valuable advice.

P.S. Bufus, thanks for the trick on finding the 1st order reflection. I try it tomorrow