Krell_man, you may be thinking of the oft-repeated admonition that you shouldn't hear the sub as a separate entity. It should extend the bass of your speakers transparently, without calling attention to itself as a separate driver.
Or, perhaps, to the fact that few instruments have musical fundamentals below about 40 Hz, although some do, e.g., pipe organs go all the way down to 16 Hz.
In any case, you can hear the lowest frequencies a dynamic sub puts out, at least down to 14 Hz (the shibboleth about being able to hear only down to 20 Hz apparently isn't true). Also, depending on the installation, subs may be used through the entire deep bass range, e.g., crossing over at 80 Hz or even in the midbass. This is typically true when they're used to supplement small satellite-style speakers that are specifically designed for use with a sub.
Finally, subs produce a lot of out-of-bandwidth signal because the crossovers aren't completely sharp. Forex, if they cross over at 80 Hz, you'll still be hearing them at 160 Hz with a four-pole crossover or even higher.
Bottom line, they're very audible.
