How do you determine what to set your subsonic filter on the side of your amp to? I have 2 15's in a a duel slot port box. Any help would be appreciated.
depends on the Fs of the subs, and what you have the box ports/slots tuned to (frequency.) I'd say between 16-20Hz off hand.
You want to set the SSF to about one octave below the port tuned frequency. That allows for some roll-off below the tuning freq, but still protects the sub from damping loss.