> When you mention using HPF to cut noise, I think of handling noise or wind.
Well, using a low-cut filter for wind noise is often a devil's choice--the noise tends to be so broadband that you often have to cut the whole low end just to keep the wind noise down to an acceptable level. You really need to prevent wind noise (i.e. at the microphone), rather than recording it and trying to fix it later. It's not a minor problem in sound recording; it's a fundamental one, and calls for a fundamental solution, not a cosmetic one.
> If someone is interested in just attenuating bass during an ambient recording on a fixed stand, would 6db down be a good balance between full on thumping bass and elimination of the lowend?
Potentially, yes--but it would be far better to make this type of decision afterward in post production. You only have a small number of available choices built in to the preamp; you can't be fiddling with the jumpers during a live recording and guessing which setting to use based on headphone monitoring.