Made the above statement several years back based on some inaccurate observations.However, putting the extender on BR or the horn cabinets has little effect below about 70 Hz.
Better measurements proved all types of cabinets forward level increased with boundaries placed beside them down as low as the cabinet response went.
http://www.diyaudio.com/forums/subwoofers/204472-multiple-cabinet-combined-response.html
Art