Is it a general practice to highpass at exactly 20hz and cut it off? or highpass 25hz? during mastering?

Or just leave that frequency range or don't highpass it?

 

This Depends on your Sourcematerial, use a Frequencyanalyser and your Ears... but in General a Highpass starting at 40hz is recommended to kill the unwanted Low Frequencys.

 

Try highpass around 30-35hz, with the best 12db or 18db filter you have. Flip it on and off and listen super carefully to just the low end part of the track at a loud volume. You should be able to hear changes this way, what the highpass is doing. The question you need to answer is: did it become tighter,cleaner, nicer? Or did it just become a little bit weaker?

If the second, get rid of the highpass. If getting rid of it still doesn't make you happy with the bass, try a 6db highpass and sweep it up from the 30hz position, while listening the same way to how it eats up the lows. There will be a certain point where it gives you a good balance of subsonic taming and nice bass sloping, which is what you might decide to stick with after making the same standard comparison as above.

Doing a 30hz highpass "blind" without even listening is OK in mixing where you're just removing the subsonic trash on individual live recorded instruments or individual synths, stuff you know you definitely don't need in your track because you plan to have your own bass occupying that frequency area. It's a bad idea to do it blindly for "mastering" or whole track finalization, because any HP filter you put on has that effect where it can make the bass weaker and you could miss it at first, if going blind.

 

Also remember that every filter is resonant in some manner and that no filter does it work for free.

Unless you need a filter to clean up something in which case it would seem to me that 30Hz a somewhat low frequency for you to know you need a filter there. When you put a filter at 30Hz you are going to hear the effect of the resonance of that filter and not the filter itself in most cases as infra-bass at the range is as much about feeling as it is about hearing.

You also need a solid room and solid loudspeakers to even get a clear idea on what's going on in that boundary layer bellow 45Hz where feeling and hearing crossover.

In the end though, to filter or not to filter: it will come back to same answer you will always get in these types of scenarios... does it sound better with or without and can you actually hear what you think you are hearing; is your brain fking with you again. I know I can't trust mine - it's been telling me lies me about everything every time I let my guard down, for pretty much my entire life.