Kevinzoe your proposed solution of using a Marchand to remove low bass from your mains to send to the subwoofers will work and is a very good idea. I am doing the same thing, but using a Behringer until my Marchand arrives.
BTW, baffle step compensation, notch filters, group delay, etc. are available for Marchand crossovers. They are drop-in cards.
BTW, baffle step compensation, notch filters, group delay, etc. are available for Marchand crossovers. They are drop-in cards.