the HPF is a nice thing to have. It becomes necessary when you play 5 string or in a low tuning because those very low frequency can add a lot of mud to your signal.
This is kinda correct - the real issue isn't that low frequencies themselves add "mud", but that putting low frequencies through speakers that can't handle them creates odd harmonics, which are heard as mud.
If you're playing though very capable PA's (lots of tour grade subwoofers), a high pass filter set really low - say 20 Hz, helps get rid of subsonic signals (when you pull on a string before you release it, there is subsonic content there) that can use up power in the PA, but you don't need to roll off the actual intended signal of that bass - down to 31 Hz-ish for a low B on a 5'er.
If you're playing through a bass amp, then you need to set your high pass considerably higher - as odd as it seems, bass guitar cabinets don't work down to the lowest frequencies that our instruments put out - they're designed to be small and efficient, which means they give up those lowest notes. And yes, I mean 4 or 5 string basses - 41 Hz (low E on a 4) is below what bass guitar cabinets do well at decent power. Where you need to set your high pass in those situations depends on your gear (including the bass), and how loud you play.
One of the secrets of bass gear is that bass amps all have high pass filters in them. There was one exception - Acoustic made a line of "DC coupled" amps many years ago, that quickly vanished from the market - every note you played, the speakers jumped from the transient, and they didn't get very loud at all without clipping, because they didn't have a high pass filter in them. Anyway, Whether or not the filter in your amp is set where it needs to be for you is a good question.
