Bass cut or high pass filter, either on a preamp or a mic, attenuates a selected frequency range, usually somewhere between 40hz and 180hz. What you use it for depends partly on taste, and partly on what frequency band you're cutting. A lot of people, including myself, will cut an acoustic guitar, especially a jumbo or dreadnought at 140hz or so, to decrease "boom" the overloading of the channel with bass as the mic approaches the soundhole. It allows you to position the mic a little closer to the soundhole, and produces a different kind of sound. A lot of others will use bass cut, say 40hz-80hz, kind of as an electronic shock mount on a vocal mic. Most singers have very little output in the 40hz range, even harmonic undertones, so it doesn't do much to the vocal track, but attenuates low frequency handling noise, mic stand bumps, and dumb foot tapping.
You can use bass cut on anything that simply produces too much bass or anything that *never* produces bass, such as a piccolo. If you've got bass on a piccolo track, you can be pretty sure it's unwanted noise. What the hell, why not cut it? I've seen it used very effectively on a 3 mic setup for piano. A matched XY pair, and a third mic, omni, bass cut, and blended to taste. It was lovely.-Richie
