Mine goes into the high teens before starting to drop off, and from there on the amp's SSF kicks in. Without a SSF I'm not sure how low it would go, but probably about 16-17hz before dropping off. At that point you really need to turn up the volume to feel it anyway, and music never goes that low, so it doesn't matter too much.
Unless you are going to drive around listening to test tones all day, i doubt you will ever find a song with a 15 hz note. You would be hard pressed to find 10 NORMAL songs that have notes under 30 hz.
And just in case you didnt know, the SSF that Sr20dem0on was referring to is short for Subsonic Filter
15hz is prettymuch inaudible. I cannot hear below 20, and as a general rule most people cannot. I have always found movies to need the low end more than audio. The Matrix has a lot of very low end bass notes, but without a decently good sub turned up high (Def Tech Supercube I in my instance), you will not even hear it. Songs in such low tones would be prettymuch useless, especially considering the majority of listeners would not be able to hear the bass beat at all.
also to hit that low you need a good combo of both amp and sub. i had a duel amp as of yesterday and many many times when the bass starts going down it would just cut off! now i have an alpine amp and it reproduces everything (within its limit of course) with a very tight hit. :thumbup: now i just need to get a new cone and it will be even better!