I'm designing a crossover for a center channel mtm speaker. I'm using an 8ohm tweeter and two 4ohm woofers wired in series to create an 8ohm load. When doing my calculations for the low frequency section of the crossover, do I use the 4ohm rating of a single woofer or do I use the 8ohm rating for the two woofers together. I'm not new to speaker building but I am new to crossover design. Any help would be appreciated.