I have a 15" orion h2 in a ported box. I have a mtx 1501d ampto push it. the amp doesn't have a subsonic filter on it. I want to know if a 20hz high pass fmod will do good for my setup. if I remember, that highpass should take out everyting from 20hz and lower, which is what I would set a subsonic filter to and it would take out the same frequencies. Am I right? thanks for your help.
20Hz will do the job. Generally I suggest using an infrasonic filter set about an octave below the frequency of the port tuning. That allows for the roll-off of the box so you're still getting pretty much full audible range from the subs. If the box is tuned much above about 35Hz, I'd step the filter up to around say, 25Hz for a 45-50Hz tuned box etc. one octave down is equal to half the tuned frequency. so tuned to 40Hz, you'd want a 20Hz filter.