You could have just asked when you got to my house...
You are not looking for a low pass filter, - you receiver is doing that as a crossover point... ie 80hz sub xover to the speakers.
What you are looking for is a High Pass filter, so nothing below a certain freq is sent that polk...
Personally I don't believe in mid bass modules, get a good competent sub and it should handle all the freq's properly.
The
Behringer MIC2200 is probably one of the least expensive I know of for this duty...
And it just so happens, that when I installed my new subs, I no longer need the one I have.