What's my subsonic filter do, and how should I set it? Also, I read about my built in x-over, and the ranges are from 50 -120Hz and 50-750Hz, the problem is, I've set it on the latter of the two, and I don't get the shaking I used to, but the range simply got higher on one end, and didn't lose the 50 Hz range, what's with that?
A subsonic filter is designed to keep out low frequencies, primarily from subwoofers. Usually you will use this when you have a subwoofer in a ported box, you use it to limit excursion below tuning. Typically I set it to around 1/3 octave lower than the tuned frequency of the enclosure. What kind of amp do you have? You've likely set the crossover to highpass instead of lowpass, cutting frequencies below 50hz and allowing anything above to pass.
TB, on A8000T amp, it has LP and HP. 50Hz - 750Hz is the HPF. That basically cuts off bass. So if you had it set to 80Hz, you'd be losing out all the low bass. That mode is used for mid bass not subs. Switch it to LPF and set it around 80Hz - 100Hz. That should give you lot of "low" bass you're looking for. Unless your sub is ported, subsonic filter isn't really required. If you want, you can set it to 20Hz. Anything below, you won't hear anyways.