IMHO the explanation of this phenomena (that incidentally
I was thinking about for few days now ) folowing the same
observation as yours is the following:
as Limabean observes, the lower frequencies have a much longer wave lengh that the other frequencies and in order
to reach a maximum has to travel accordingly that distance.
I understood that a full range trasducer with full bass
extension down to 20HZ ( not necessarly from recorded music,
but from an artificially generated recording/sampler as
the Stereophile Test Disc 2 or 3...)will reach it's peak
at about 40 linear feet traveling from the transducer.
I have a pair of speakers claiming such 20Hz reproduction
( Apogee Scintilla ) and indeed the sound in the 17' long
auditioning room doesn't reach the same bottom as the same
sound travelling through the open door another 25' or so,
right at the end of a corridor, were everythin vibrates and
shakes in burst of energy. I might be wrong, but that's my
explanation to what I've observed.