Do you have an option to try it with all other things in your chain replaced....cables, soundcards, monitors...so you can rule out its in your chain somewhere?
Not optimistic...cause you face the same issue with headphones....
Did you check for noisegates in stomps? (Also longshot, cause not all profiles have that..)
Did you check for phase inverts somewhere? Try the mono out to rule out that. (Doesnt work If your speakers are out of phase)
Maybe someone can recommend a phase plug-in to monitor that?
