I'm not sure about the 60Hz thing, but if you have lots of signals below that then they will affect things like compression, etc. that you apply to your mix but may not be apparently obvious if your speakers can't output tones that low.

I'm fairly sure I have bought tracks with bass extension below that. In theory analogue synths and analogue recording (i.e. records) can produce a bass tone much lower than this (say 0.1Hz) the problems come with the fact that to record that kind of frequency your going to make some massive grooves in the record - extra thick vinyl anyone?

My MC202 can make my speakers wobble with such bass frequencies that I can't hear it but can 'see it' - i.e. watching the cones move in and out slowly.

20Hz is specified as the lowest tone digital encoding can re-produce (although I'm sure this is related to frequency domain, I can't see why time domain "sampling" couldn't do it). Your ears aren't going to pick up anything below that as it's also the supposed lower limit of human hearing, although your body might - i.e. you'll 'feel' it.