You are definatley in a higher class of speakers than me. Given that class, I can't imagine any speakers in that league having one note bass.
I would look at the room for issues. Has the room been treated? When you say "about" a 10 by 20 room, how exact is the about? I hope you don't have 10 foot ceilings to go with that.
Was the one note bass about 55hz? 10 feet calculates to a standing wave of 56.5hz.
The echo from the 20 foot wave would also directly affect the same 56.5hz.
I would look at the room for issues. Has the room been treated? When you say "about" a 10 by 20 room, how exact is the about? I hope you don't have 10 foot ceilings to go with that.
Was the one note bass about 55hz? 10 feet calculates to a standing wave of 56.5hz.
The echo from the 20 foot wave would also directly affect the same 56.5hz.