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''.

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.

Thaks for the help. I've just ordered Rives test CD2 and will try to update you on the proggress.
BTW, on the Rives room simulation page, my speakers
( Paradigm Studio Reference 100v.3) are not listed, so I used the closest in terms of the size and a number of drivers speakers- Triangle Celius. Not sure how it affected the accuracy of his measurements, I don't think it did, b.c., changing speakers in the virtual setup didn't change anything at all (almost). Regards.