I've been playing around a little with ideas on how to augment music with ultrasonics/infrasonics. On the high end I fancied including features in songs that only young 'uns could hear, along the lines of the teen buzz ringtone. Unfortunately I've blown one of my tweeters messing around with this, so for now it's on hold.
But there's also a lot of interesting stuff at the low end - notably The Ghost in the Machine which is a proposed explanation for some ghost sightings, whereby there's some infrasonics (caused by wind through a room or whatever) that hits the resonant frequency of the human eye, around 18Hz according to NASA (pdf) causes weird visual artifacts. Of course I couldn't resist playing with this idea, but the speakers I've got won't really go that low. But that led to thoughts of sync'ing modulating LFOs to the BPM of songs. For example, if the song is running at 135 BPM that's 135/60 = 2.25 beats/second. An eighth note at that tempo (in 4/4, i.e. 32 notes in a bar) = 2.25 x 8 = 18Hz. Heh.
It just occurred to me that being not that far off actual subbass, this kind of frequency range could tie the musical note frequencies to the BPM. So I got some note freqs and bunged them in a spreadsheet. Results here (rounded):
(Note that these note freqs are generally considered below the usual quoted hearing range 20Hz-20kHz, I think in MIDI terms that C is 2 octaves below C0)
I would hypothesize that if you're using some really low bass, then the overall sound may be enhanced by choosing a key/bpm that is close to an integer ratio of those values, e.g. if you're going for some laid-back reggae then maybe 92 bpm in the key of G might be most effective.