Room Acoustics

You should start with a test CD, Stereophile test CD 2 or 3 have test tones. You can use this in combination with a Radioshack SPL meter and graph your room/speaker response. Sean did a very good post on the corrections for the Radioshack SPL meter--it's not linear. You should be able to do a search and find it, but if not I can repost it. I copied it for future reference it was so good. Most likely you are getting a bump around 40Hz from the 15 foot wide wall. That will make the bass bloom---way to much. Unfortunately, problems at that low frequency are not well compensated for with room acoustics. They generally only work at 80-100 Hz and above. They can help--but they won't be the cure. The cure in severe problems is active eq. A notch filter where you have that bump. I know it almost sounds like heresey to use an eq--but there are audiophile grade eqs (TACT, Sigtech) and you might be able to get by with a simple yet high quality notch filter (not the sophistication of the TACt or Sigtech--but might do the trick).

80 to 90 Hz is your floor to ceiling dimension. You already have carpet, but it might not be the best for absorbing these waves. While I stated that active EQ might be the way to go (I had to do it), it's always good to try to do as much as possible without active EQ. As another poster said--make sure everything else is right first. If the short shag carpet is not very dense, it might not be absorbing the bass waves. You may consider replacing it with denser natural fiber carpet. (probably cheaper than another pair of ASC bass traps).