06-20-12: Paulsax
I seem to recall that a typical electric bass guitar drops to mid to high 30's on the open low string. For some reason 38 is sticking out in my mind. lots of fronts will do that but the discussion on spl above my explain why the sub helps out.
If you check this chart, you will see that the low E of a 4-string bass is 41.203 Hz. For a 5-string, the low B is 30.868 Hz. Maybe that's what stuck in your mind (with a little modification). Or maybe you were thinking of Eb, which is 38.891 Hz.
Your second part is definitely true. You can't count on many speakers to make meaningful bass below 50 Hz. Many start rolling off around 80-100 Hz. Of course this depends on speaker placement, room-loading, etc. And more expensive speakers definitely go lower.
Still, if Wilson Audio tours the Alexandria XLFs with a pair of Thor subs, I'd say just about *any* loudspeaker would benefit from the right pair of subs properly blended.