Steve, welcome to Axiom. Looking at the numbers for your equipment it appears that the K100-SW will add relatively little extension to the already good bass of your M80s. There should however be some reinforcement in the 30-60Hz octave available if you set the RP15 switch to 60Hz(incidentally, there's no need to use the FP15 bass boost switch, as the M80s have plenty in that area, and it should be left off). So, the Outlaw suggestion to use the RP15 switch is correct, but not for the stated reason, since subs have no "similar circuits", as was discussed in the previous thread which Dan linked.
There's no bypass on the low-pass control on the K100-SW, so it should be set to the highest 160Hz setting to take it out of the way of the 60Hz crossover which the RR2150 circuitry will be doing.
