I agree to run the monitors and sub separately, but the low pass will not affect the monitors the other way. The output speaker connectors on the sub will either be full range or a high pass filter, which is generally a fixed frequency, and not affected by the crossover adjustment.
Subwoofer connection to integrated
i have an integrated tube amp that has no preouts, just 4 inputs. this is connected to some monitors that are rated down to 55hz. i am looking to buy a sub and going the speaker level input route seems the only way to go. i also want to cut the monitors off at 80hz and let the sub do the work. if i run speaker level from the integrated to the sub and then off the sub to my monitors{sub would have a 80hz high pass filter coming out}, would there be any issues doing it this way? are there any other options?