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).

First make sure that it's NOT the equipment. Using a test CD monitor the AC voltage at different frequencies. If that's OK check the speakers. This is a little tougher. Best would be outside if you have long enough cable, otherwise in the largest room. Using only one speaker and an SPL meter monitor the level at different frequencies. The meter should be very close to the speaker. Also check with Avalon about any impedance anomolies at the offending frequencies. Also the resonant frequency of the speaker / spike / floor combination could be an issue. Get the exact measurements for the room and calculate areas that would be likely to give you a problem. If the room measurements are OK than it's not the room. Bad floors are a possibility too. Contact me back in a few days after you've tried some of my suggestions. By the way, a good SPL meter and all purpose electrical meter are indespensible. Also get all of the cheap test discs that you can. You never know when one will come in handy.

DEar MES

What is your wall made out of. If it drywall on stud it usually acts like a drum near the frequency you are describing.
If you do have that kind of drywall simply thump the wall in various areas and listen to the thud. Compare it to the pitch of the blooming artifact from the speakers. That might be helpful.
This is similar to the floor artifact you are describing bu thas a pitch nearer to what you are describing

jd