I think you mean PFL which is Pre Fader Level. Many mixers (for instance Mackie CRs and Allen and Heath MixWizards), will have a feature where when you hit the appropriate button (PFL, solo, etc...), the input level of the channel (or channels) selected, shows up on the main meter (usually when there's only one master meter, and the channels might just have clip lights). So for instance, while dialing in the trim for your kick drum mic during soundcheck, you switch that PFL or solo button on so you can get a more accurate idea of what the gain is doing.
With most modern digital mixers, this isnt necessary because there's usually a meter for every channel somewhere in the UI.
I would also add that pure signal level will only get you so far in terms of matching your different sounds. You really need to listen to each sound alone and in the band mix. Signal level can get you in the ball park though, and you probably want to keep the signal level range to maybe 10 db or less. But also keep in mind a little goes a long way. Sometimes 1-3 db is plenty to compensate for a "quieter" sounding sound that you want to be at the same PERCEIVED volume as "louder" sounding one (things like EQ and compression can affect this).
Guitarists seem to like about 10-15 db of boost for solos, and it completely ruins their whole sound because they were probably already too loud on the rhythm sound, and completely out of FOH, and then they kick in the boost and it just becomes obnoxiously too loud, but there's nowhere to go with it. And yet the sound is going to change as you move around the room because they're completely out of the PA and their amp is much more directional. If they were at a reasonable stage volume to start with, and then had maybe 3-6 db of boost, even if the boost ends up being too much, I would still have room to bring them back a little bit and set them nicely into the mix.
So yeah, keep in mind things like phasers, filters, octavers, compression, and of course any sort of gain pedal is going to change both your signal level and perceived volume. So matching signal level is a good starting point, and then you can try to match perceived volume (with adjustments as appropriate (ie you might want your grindy fuzz sound for when the band is really rocking out to be a little hotter than your normal sound).
With most modern digital mixers, this isnt necessary because there's usually a meter for every channel somewhere in the UI.
I would also add that pure signal level will only get you so far in terms of matching your different sounds. You really need to listen to each sound alone and in the band mix. Signal level can get you in the ball park though, and you probably want to keep the signal level range to maybe 10 db or less. But also keep in mind a little goes a long way. Sometimes 1-3 db is plenty to compensate for a "quieter" sounding sound that you want to be at the same PERCEIVED volume as "louder" sounding one (things like EQ and compression can affect this).
Guitarists seem to like about 10-15 db of boost for solos, and it completely ruins their whole sound because they were probably already too loud on the rhythm sound, and completely out of FOH, and then they kick in the boost and it just becomes obnoxiously too loud, but there's nowhere to go with it. And yet the sound is going to change as you move around the room because they're completely out of the PA and their amp is much more directional. If they were at a reasonable stage volume to start with, and then had maybe 3-6 db of boost, even if the boost ends up being too much, I would still have room to bring them back a little bit and set them nicely into the mix.
So yeah, keep in mind things like phasers, filters, octavers, compression, and of course any sort of gain pedal is going to change both your signal level and perceived volume. So matching signal level is a good starting point, and then you can try to match perceived volume (with adjustments as appropriate (ie you might want your grindy fuzz sound for when the band is really rocking out to be a little hotter than your normal sound).