Another thing, my sub is in a ported box, so the Subsonic filter at OFF is the correct position. But if the crossover only goes down to 30 Hz, then there shouldn't be anything going to the sub lower than that. So why the need for the filter? They say that the subsonic filter removes frequencies not audible to the human ear. These frequencies are only robbing power from the amp. So I guess the new question is, how do the subsonic filter and crossover relate?

No, with a ported box you do want the sub sonic filter on.

The normal (low pass) filter will give the sub anything below the filter frequency (say anything below 100 Hz). But your ported box is tuned to a specific frequency (say 40 Hz). If you play content that is "much" lower than that you risk unloading the sub, because the cushion of air that prevents the coil from slamming back and forth between the ends effectively has time to leak out with those lower frequencies and the air then no longer offers any resistance to the coil/cone.

You can compare that to a leaky football - punch down on it and it offers a fair resistance because the air can't leak as fast (higher frequencies) as you punch the ball. Now instead slowly (this are the really low frequencies) squeeze the football - the air will offer very little resistance as it now has time to leak out through the hole (port).

The sub sonic filter is actually a high pass filter, but with a very low frequency.
So, with both the sub sonic filter and the low pass filter active, the sub will see signals above the frequency of the sub sonic filter (15 or 30Hz) but below the frequency of the low pass filter (say 100Hz), ie frequencies between 30 and 100 Hz.

/Michael