What's Subsonic Filter on an Amp?

A subsonic filter will attenuate frequencies so low that your subwoofer cannot reproduce them OR that humans are incapable of hearing. Subsonic filter is also a very important feature for vented enclosures.

What do I set my subsonic filter to?

Each octave up is twice as many Hz as the previous one, and dividing any frequency by 2 takes it one octave lower. Therefore, to get your subsonic filter setting, take your subwoofer's tuned frequency and multiply it by 0.75. So, if it's tuned to 40 Hz, set the filter knob to 30 Hz.

Is a subsonic filter necessary?

Subsonic filters are useful, but not a total requirement. It really depends on where the tuning frequency is vs how low the program material goes.