"Most subs have a built in crossover that will limit the base going to your main speakers. Do a search for subs and you will find many with built in crossovers"
True that most subs have a built in crossover, however, it is generally a low pass filter that has no effect on bass to the main speakers. A few do have a high pass filter that is necessary to limit the bass to the main speakers, and it is usually a line level connection. Speaker level high pass filters are rarely used nowadays.
I don’t understand how bass could be a problem. Four woofers in large cabinets that are spec'ed to 25 Hz with 400 watts per speaker, since they are 4 ohm. I’m thinking there is a problem elsewhere.