15hz is a pretty good goal for extension, if it's in limited quantity.
Extension below is honestly not as exciting as it sounds. 15-30hz is where the real meat of that sensation of "weight" is. Below 15hz, there isn't so much sensation to be appreciated. The effects on the room on the other hand, are quite palpable sometimes. Will this affect your enjoyment? Maybe. After a while the effects become less interesting and more like a cheap parlor trick.
Don't get me wrong, I'm not poo-pooing full bandwidth extension. Just saying that producing it might not really add to the enjoyment of either movies or music....as 15hz and up surely will.
As @SME mentioned, you can still build a riser with no drivers. That is a thing people do.