I'd say there are exceptions to that rule, allbeit using some pretty serious engineering;

How about a sub measuring little more than a cubic foot and with a 10" driver that can reach down to 18Hz (anechoic) and yet with harmonic distortion typically below 1%!  The 15" version of the same sub has an anechoic response down to 15Hz.

According to your theory, both these would only qualify as mid-bass producers.

Matt.