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usualy I turn the amp l/r up full, then set the input so the it isn't driving the amp too hard - use the main L/R fader on your mixer to set the input level to the crossover/amps
thats pretty much the common practice, but if your amps have more than enough juice for the venue i'd recommend turning them down a bit so you dont end up setting all your mixer faders to half way down or whatever. plus if your amps/eq's/xovers are of the budget type they may be noisey, and by feeding a strong signal into them you'll mask most of the noise coming from them, and in turn also reducing the possibility of feedback by a bit too.