i played a 20hz test tone on a piece of crap advent sub in a bandpass box and it sounded like a helicopter taking off, i don't know if thats what its supposed to sound like or if it was the crappyass sub i was useing
You were probably listening to distortion. 20Hz bass note should be something that will not be heard, but felt. More like a "wind" vibration. If you ever look at a sub that can actually produce 20Hz note, you'd notice that it's movement is very rapid while producing very low audible bass. The notes that your ear picks up are usually 30Hz and above. Anything below that just shakes and rattles windows and chests.