My thinking is that low frequency bass wavelengths such as those produced by guitar and bass amps are many feet long for one full wave. How this corresponds to how we perceive the bass sound in a small room or from headphones, I'm not sure. Of course, not being unidirectional, a lot of the waves are not going to have the space to fully form their waves before being reflected and atenuated. They are bouncing all around at high amplitudes reinforcing each other and cancelling each other out.

Here are the frequencies and wavelengths for the strings on a 4 string bass +/-:

String – Frequency - Wavelength
G - 97.9989 Hz - 11.531 ft.
D - 73.4162 Hz - 15.392 ft.
A - 55.000 Hz - 20.545 ft.
E - 41.2035 Hz - 27.425 ft. (329.098 in.)

The above wavelengths will occur in a free space, a space free of obstructions, walls, ceilings, people, etc.

Since our amps are located near walls, have open backs, are in enclosed rooms, etc., there are going to be a lot of powerful high amplitude sound waves bouncing around reinforcing each other and cancelling each other out, as the case may be in any given space. This is what I think Dee is talking about - the situation where you have sound waves bouncing all around and affecting what our ears perceive. Therefore, experimenting with the placement of the amps and speakers, aiming, etc., can have a big effect on how we hear our music. Because of this recording engineers go to great length to try to deal with this problem and the bass frequencies are particularly problematic for them and they go to great length, in some cases, to try to get it right or at least "better" sounding.

Also, it is seemingly confusing why we can hear a 40HZ bass note correctly thru a set of earphones. The amplitude of the sound wave at 40HZ coming to our ear from the headphones is much lower than the amplitude coming out of a bass amp. But the wavelength is still about 27 feet long, so our ears and brains must be processing that E note somehow so that it sounds right, even if the note is very short in duration such as one tenth of a second - it still sounds right.