You used to be able to buy a subharmonic synthesizer from Radio Shack of all places. I don't remember what it had for controls, but the jist is that if there was a frequency at 24Hz, it would create a corresponding signal at 12Hz at 6dB lower...mixed back in with the original. It would do this for all frequencies below a certain point, creating a harmonic lower in frequency and level than the originals. This was before subs were even talked about in HT.... and before HT itself was ever really talked about.
I've often wondered why a product like that is not re-introduced for the current crop of very capable subs.
Edit: I should have researched it before answering:
dbx still makes one!
