Foam bass traps and diffusor panels

Correction: my room dimensions are 21'5'' x 16'8'' x 8'. I have my speakers positioned according to formula-
Room width x 0.447= distance from the back wall;(7'5'')
Room width x 0.276= distance from the side wall (4'7'')
According to Rives these distances should be 5'9'' and 4'4'' respectively.
My sitting position is in the apex of the triangle with all sides of equal length. Initially it used to be closer to speakers, with the same bass response pattern. Moving it farther away didn't change the sound much, with the exeption of the subjective impression of sitting in the middle of the concert hall instead of the front raw.
Actually the ideal width of my room with the same lenght should be 13'3'' vs. my 16'8''.

Now I understand more of your problem. You have undoubtedly dimensional induced bass standing waves. Have you done a check with a sound level meter (Radio Shack) with a test disc (Rives is adjusted to the RS meter) to find out where your bass rises and suckouts are? And, more importantly how broad or narrow they are? When you measure at the listening position remove the chair and walk back and forward a bit and see what happens (ditto for the speakers) - this will help you ID which frequency is, or might be, problematic because of seat/speaker positioning, and which frequency is a room node problem (as that won't change much with speaker/seat changes. Its the pits - I've got a 9db rise at 32 hz that I just can't get rid of so I feel for you.

Thank you, Newbee! I do have an SPL meter, but haven't gotten around to the test disc yet. That's definetely a next step. I'm also waiting to complete some reconstruction project- building a new wall, the left side of my basement is open to the adjacent part. So I don't want to make any changes now until I've completed the wall, which definetely should change the whole picture.
Wanted to hear your comments, though, on the speakers placement measurements- should I go with the Rives or with the above formula? Regards.

Well, FWIW, Rives places my speakers within a few inches of where I had already located them by ear/meter/disc - I didn't move them. His program had the seat a foot more forward than I set it, coming much closer to an equalateral triangle. I have found with most speakers I prefer something closer to a 11 to 10 ratio with the speaker being further from the speakers than the speakers from each other. For me the process was (1) finding good basic imaging, (2) realigning seat and speakers to get the bass as flat as possible, then (3) fine tuning the imaging over a period of many months - no more than an inch at a time, with a considerable intrim period between movement. Slow gets it done right - at least thats what the turtle thinks. Let us know how it works out.

I agree with trying Rives formula and buying his Radio Shack corrected CD. Place your DB meter on a tripod at your listening position. Write down your db output at each frequency from about 32hz-200. Keep track with each speaker movement. Based on your positioning, I would guess that you would want your speakers closer to the back wall. Notice which frequences change, what direction, etc. Move in small incriments. Distance from side wall will also change bass characteristics at the listening position. In my room (Approx. 20x28x8.5, non-parallel walls and ceiling) my speakers are about 11 feet apart and 14 feet from the listening position. They are closer to side and rear walls than yours. I like the less "near field" quality of greater seat distance (mid-hall). As a general rule in my room moving speakers farther from the rear wall increased 60hz, and decreaased 50hz. I found a nice balance by watching the meter as I moved speaker position foreward/backward. I also found reduced 60hz as I moved the speakers closer together. Additionally, listner position can change any of these frequency measurements by huge amounts as you move a matter of inches. Rives is great to work with and you might want to consider using his bass EQ. He is designing a rooom for my brother now and he really seems to know his stuff.