Are you seriously complaining that your speakers didn't have an audible signal below 20Hz? The human range of hearing is from 20Hz to 20kHz so everything you hear below that is actually just noise, most likely the flowport creating vortices and thus creating higher harmonies that you are actually hearing..Or your speakers are actually starting to break down creating distortions ie higher harmonics... edit: just tested this and the Youtube compression is the culprit here, not a true test, use the link that i provided..
And since this is not a balanced curve, the 3kHz should be very very loud indeed, that's where our ears are the most sensitive. You can check this by using signal generators, here's one online:
http://onlinetonegenerator.com/
NOTE!!!! Pure sinewave is very loud as it does not have any harmonies, it is the most basic form of sound, think of it as the atom of sound. It WILL destroy speakers easier than you will notice as the level seems to be lower than it really is. This is doubly true below the resonant frequency of a reflex type box, that's the one normally used, check this before testing:
http://en.wikipedia.org/wiki/Bass_reflex if your speakers are built this way (99,9% change that it is...) So you have been warned, set your volume accordingly.. Set the generator to 1kHz, tune your sound levels and don't touch your volume pot after that.. Good luck!