If you use a dsp, you can lower the level, and if you connected them correctly you can use your Hu fader.
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How would I use the fader if im using the headunit/processor in a 2/3 way mode. No front and rear anymore, only high and low, and sub low.Sure, if you've wired your stereo in a way that renders the fader unless. Ohterwise use the fader feature that pretty much all stereos have.
I do think though, that if you have the rears loud enough to make any difference, then they will be having a negative impact on the stage. Whether or not that matters is up to the listener.