It seems that with the low pass filter, as well as the high pass filter, The lowest DB level it offers you is 6dB. I would like to try four or a different number. Is it possible? Thanks!
The high pass and low pass filters are Butterworth filters. A Butterworth filter has a whole number of “poles”. A single pole Butterworth filter has a 6 dB per octave roll-off. A 2 pole Butterworth filter has a 12 dB per octave roll-off (and so on). If you need less than 6dB per octave roll-off, then you need to use a different kind of filter, for example, the “Filter Curve” effect https://manual.audacityteam.org/man/filter_curve.html
These settings produce around 3dB per octave low pass at 1000 Hz:
I would like to try four or a different number.
Why? Do you have a specific application in mind?
Sometimes addressing a question like this in a vacuum isn’t useful. The poster may misunderstand what the tools do and is narrowing the question to the wrong tools.
“I want to [ real question ] to my sound.”
If you don’t have an actual sound job, that can create other problems. Audacity doesn’t do scientific measurement or data collection jobs very well. It’s an Audio Editor and if there is ever a toss-up between scientific accuracy and sounding good, Audacity will always pop for sounding good, leaving the scientists wondering what happened.