Connect the front outputs to an unused input of the preamp or use the pass through function of the pre amp has one.
If no pass through, then select a volume point that you will use for unity gain, make note of that position or vole number ( you will always use that number or volume point every time you use your processor with the preamp) adjust your front speaker levels to match with each other il using your spl meter. Done.
If no pass through, then select a volume point that you will use for unity gain, make note of that position or vole number ( you will always use that number or volume point every time you use your processor with the preamp) adjust your front speaker levels to match with each other il using your spl meter. Done.