Levels of instrument sounds, when humans are listening, are subjective and very much depend on the conditions surrounding them. (A nice way of saying, you'll need to set them, generally, by ear.)
If you make a tenor sax sound the same exact (meter) level as the piccolo sound, I bet dollars against donuts that the piccolo will sound louder in most music ensemble situations... reason: less competition in its frequency range. Ever notice how they cut through the entire marching band... the littlest instrument is by far the most clearly heard! (Not because it is loudest, it simply has no competition in its range!)

There is no magic bullet for setting levels evenly. It will always depend on what else is sounding, and the conditions of the room. Bass sounds when outdoors get diffused and lost easily because they spread out and don't have surfaces to move. So generally you need to adjust your selected sound based on the instrumentation you are playing along with and the environment in which you're performing.

That said, no meter reading will be totally accurate to what you hear, because it cannot judge the content of the rest of your band, and they have a lot to do with what you are hearing and not hearing. (As I learned early in my music career), that's why you have rehearsals.

You can use the Master Effect set to a Compressor/Limiter - used properly you can even out the overall output level. This will have the overall output under control... it will allow softer sounds to be raised and louder sounds to be reduced.

Thank you very much 😉 Ill try to record a live session (we use electric drum) and work it out. Best regards!!!!