The high-pass filter is at 80Hz. I recommend you engage it all the time - except perhaps if you're recording the low end of a piano. What a high-pass filter does is allow all the frequencies about the cut-off mark to "pass" through. In the case of the 4047 all the frequencies below 80Hz are not allowed to pass through. What the high-pass filter does in practice is not allowing the kinds of low frequencies that would cause rumble or low end ambient noise to come through the mic. It's actually a good idea to use a high-pass filter somewhere around 80-100Hz on everything except bass, kick and low piano or synth notes. Other than that, there's no frequencies below that point that help define many instruments such as voice, acoustic guitar, amps, drum OH's, most percussion, brass, strings [ except for bass and maybe cello ] And leaving in those frequencies can contribute greatly to having a muddy mix that's not defined. There's some exceptions: If you have a Marshall stack you might not want to use it if it turns out that leaving some of the lower frequencies will give it more body. And you can also decide that in the mix.
You'll also find that using a high-pass filter will give you more headroom for what you're recording because low-frequency rumbling that can take up headroom won't ever even make it past the mic.
