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!!
Room Acoustics
Suggestions for this annoying problem appreciated.I get an annoying room resonance in the low mid bass that just excites the hell out of the room in spite of speaker placement, different amps, and corner treatments( 4 corners with ASC super 16's to the ceiling), curtains over sidewall windows etc. Speakers are Eidolons, room is 15wX22dX9h. Short shag carpet, speakers on apex couplers. Sound is 99% stunning with that occasional Dave Holland bass note that just won't quit. I think it's possibly the floor, pier and beam over a finished rec room in basement. I'm considering putting the Edolons on granite and maybe having someone professionally check the room acoustics. Is this related in any way to the downfiring port of the Eidolon?. Thoughts appreciated including any suggestions as to someone coming out to test the room. Thanx in advance.