Remember, the FAST idea actually means "Full (Range Driver with) Sub Transducer altho most of us just use a bass driver to avoid bass frequencies (<120Hz) coming from the FR driver with the obvious benefits - this has drifted up to about 150Hz in many cases but still is an approximation of FAST
In contrast, the subwoofer frequencies are below 80Hz (sometimes <60Hz) as this is where the bass become 'unidirectional', assuming high filter slopes
Just to be a bit pedantic, most room acoustic standing wave problems arise primarily in the 50 - 80Hz area and secondary interactions up to about 2 octaves higher (mainly in smaller rooms) - 8ft ceilings can be a big forgotten source of problems
... my 2 cents!