/HPF & \LPF
The idea of using High Pass Filters has received a lot of attention in the last few years. The idea is to remove unnecessary subharmonic rumble from most or all of your audio tracks.
I am definitely a fan of getting rid of everything below around 25-30hz on the Master Fader while mixing and also using custom HPF settings on many individual tracks in both the tracking and/or mixing stages.
The amount of filtering and the cut-off frequency depends mostly on weather it's an organic or electronic source. I find organic sources benefit greatly from filtering out the rumble especially when lots of cardioid microphones are adding proximity effect in that zone.
I filter guitars below somewhere around 80hz and organic hi hats and cymbals much higher around 180-250hz. Vocals depend on the singer and the sound you want but usually around 70-90hz. With kick drums and bass guitars you can be more conservative and only filter below 20-40hz as those are the two instruments you are usually providing the extra low end room for.
Electronic instruments (especially in the form of plug-ins) usually sound pretty great already and don't need as much reduction in the lower end of the frequency spectrum. Still a cut at 30-70hz depending on the sound can help free up that area for the tracks that need it like Kicks, 808's and synth bass.
When used as an effect as opposed to a low end clean-up tool, a HPF can give you lots of surreal sonic options as demonstrated on many Jamaican Dub tracks of the 70's and 80's.
Low Pass Filters are an amazing sounding synthesizer and sampler component parameter to adjust for a really great effect. When used for frequency clean up during mixing you have to understand that when filtering out the upper harmonics you are significantly altering the natural timbre of the instrument. The style of music you are creating and whether or not you want it to sound real or surreal might dictate the amount and ways that you use LPF's.
If you are tweaking a 303 style bass synth you may very well go nuts with the LPF but if instead you are trying to clean up the top end of the audio spectrum you may want to use it sparingly (maybe just on kicks and bass above 8-10khz?) or not at all. Too much reduction and you won't be able to tell the difference between your cello and your piano because the harmonics are the main thing that allow us to recognize which instruments we are hearing.
Rolling off the top end can yield some really cool sounds like cutting off everything above 1khz on a bass guitar and boosting 80-100 hz to give the bass a low end 70's reggae sound.
Having said all that, there are no rules. I'm just writing down these pseudo-science ramblings to boost my SEO 👽