Room Acoustics

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

With the Eidolons, you may want to move them slightly away from or closer to the wall. This shouldn't substitute for troubleshooting the source of the noise. I have a pair of Avalons, and they are very picky of room position. I had a similar phenomenon, and I found that some loose flooring / rafter bracing in my house was the cause. There was a weekend when we were replacing carpet that I went around my living room, sinking a 5lb box of decking screws every 3 in in the subfloor; Went down stairs while the wife was jumping on the floor and marked all of the locations where I heard noise, and secured all of the bracing with glue and screws. I put R25 insulation in the floor joists with 5/8 drywall below the listening room. All of this including the carpet cost less than my interconnects, and made the whole room much more quiet. After I was done, I heard more noise, which turned out to be a bush on aluminum siding, rattling soffet, and some tin lamps on the wall. I would guess that the addition of granite slabs under the speaker will not fix your problem, but it is not a bad thing to try... Good luck!!

J K
I have a very funny graphic idea of that day in your house with your wife jumping up and down and you drilling screws into nearly annything that moved o rmade noise....sort of right out of bugs bunny or something
jd

$4700 retail in ASC traps and still a bass problem? Is there something wrong with that picture?

....some good suggestions though from those up above...good luck.