Fred, the quoted statement appears to be incorrect, at least in part. "Near DC" is one matter, but frequencies such as 25-50Hz, which are subjected to room modes in listening rooms of typical dimensions, are another. For example, in a room with a 20' length dimension, the first mode is at about 28Hz(1130/(2x20) )and certainly varies at different locations within the room, as all modes do. The benefit of placing two subs at mode minimum locations and/or where the mode frequencies are opposite in phase and tend to cancel, would exist down to the lowest mode frequency of the room, although not to "near DC".
