Difference between low pass filter and high pass filter? – Subwoofer for monitoring forum

Hi AFiens,

I’m well aware of the difference between filters when talking regarding synths (subtractive synthesis), but something escapes me regarding the difference between high pass and low pass on subwoofers. Most have an adjustable low pass (12 db/oct I believe). Let’s say I set it to 80hz. So I let the sub load from 80Hz and below and the monitors cut out at 80Hz and load everything above with a 12db/oct slope.
And if I use the high pass, I therefore act on the monitor which cuts at 80Hz and the sub which supports from 80Hz.

But is it the same?

However, many subwoofer manufacturers offer both filters to be used simultaneously. Why ?

Also, on many subwoofers (Adam for example), the high pass has two positions: 85 Hz or “flat”. I imagine that “flat” disables the high pass. And if I use the 85Hz high pass, well… Why provide low-pass fine tuning ? I also have to put it on 85 Hz, right?!

This video from Adam Audio explains how to calibrate subwoofers. (They take the precaution of pointing out that this is only one method and that there are others).

According to them, it is necessary to take the frequency the announced lower limit of the monitor (here 38 Hz), add half an octave, add (here therefore 39 + 19 = 57) and set the filter (low pass therefore) to 57 Hz. Then stall monitors and subs at 80 db on the same injection of pink noise. No mention of the high pass filter.

Could someone enlighten me in the audience?