Dougie, Mark is correct; the fundamental low bass note at say 40 or 50Hz is just a small part of the total sound that the bass guitar produced at that instant. The harmonics of a 50Hz fundamental at 100Hz, 150Hz, 200Hz etc. are what produce the actual tone that we hear. Hearing only the 50Hz fundamental(e.g., by disconnecting the wires from the speakers and listening to the sub alone)would be anything but "musical". The harmonics are reproduced in large part by the mid-range drivers and they certainly have to be moving even on a "bass" note.
