- on one hand, the 30-40 Hz range eats headroom and is useless on `normal' speakers (home, car, ...),
- on the other hand it should sound full range on a PA with pro subwoofers and 1000 watts; for example the low B string of a bass guitar or a Moog bass pedal should come out fully.
My general approach to this is that your mix should be compatible on both the PA with subwoofers and the normal speakers. Meaning the low B string on a 5 string bass should have enough overtones to be perceived as a 31 Hz pitch note even on a system that can't truly reproduce that frequency.
So when mastering, you want to dial in some of the low frequencies so that they're heard on the PA with large subwoofers, but not too much so that they don't eat the headroom anymore than any other frequency range. Don't overcompress or try to dial too much the sub frequencies to try to make them "appear" on smaller system, or they'll become ridiculously loud and unbalanced on large PA and then they'll eat headroom unnecessarily. Balance the other frequencies so you can clearly hear the harmonics on a regular system, making your brain recreate the missing 31 Hz fundamental and identifying the note as that low B note even if you're missing the 31Hz signal.
Hope that helps.