Subsonic filter is meant so that the driver doesn't over-exert itself in a ported box where it's playing X frequency, and X Frequency isoff if possible, a sub sonic filter is meant to keep the sub from bottoming out.. this usually isnt a problem in a sealed box
It's hard to say, as it depends on the in-car response and the recording. Contrary to popular car audio belief, subsonic/ (errr...) infrasonic frequencies do waste power in a sealed sub, as well as reduce its resolution, albeit on a comparatively much lesser scale. Assuming the filter runs 12dB/octave, I'd begin listening at ~25-30Hz and then go from there. Good luck.if using a sealed box what do u set ur subsonic filter to???