A subsonic filter filters out the frequency range that is below what you can hear. If you can't hear it, it's wasted energy and isn't doing anything useful to help keep the sub accurate and controlled.

So if you have a subsonic filter, and a lowpass filter, the sub will play the range of frequencies between them because everything below (subsonic) and above (lowpass) will be filtered out.

Like stated above, those subs aren't the best choice for that amp because of their impedance. Those subs you linked to are 4 ohms each (that is their impedance), and if they are wired in parallel then you will have a 2 ohm load. That amp is not stable at a 2 ohm load in bridged mode, so your only choice with those subs would be to use the amp in stereo mode (which would only be 150W per sub, which isn't really ideal).

So yes, subs which are either 8 ohms each would be a better choice, because then they would be wired in parallel to make a 4 ohm load (total), which would give the best power output for that amp. In that instance you would need subs that each have either a single 8 ohm coil or dual 4 ohm voice coils each (two 4 ohm coils wired in series makes an 8 ohm load).