When I heard the sub doing this at close range, it turned out to be the out-of-phase modes from the second poorly-placed sub that was causing the flutter. I had another sub in my kitchen for my "B" speakers. Once I moved the living room sub to the near-field position behind the couch (which also turned out making the rest of the room sound better), I discovered that the bass now freely radiates into the kitchen where it didn't before. I was able to turn down the kitchen sub to almost nothing. And without the interaction between these two cranked-up poorly-placed subs, most of the flutter has been eliminated. The location of my main tower speakers is s horrible place for bass tones to originate. With my subs and mains crossed-over at 40 or 60 Hz, the flutter is still there because the towers are causing it. With it set at 80 Hz and fronts on "small", the sub (with better placement now) is handling all of the lower bass, and the whole room sounds better. Only a few certain bass notes have any noticeable flutter, due to having inconvenient wavelengths that don't agree with a certain dimension of the room's shape. Music, movies, everything sounds better now.