set it where you want. it's a crossover setting. it'll allow all the frequencies below that range to pass thru to the sub (provided it's the low pass filter).
the high pass filter will cut off the frequencies below the set frequency.
well i no when its at 30hz it dosnt have any bass and when i have it up higher than it seems to sound better. SO that would explain why theres no bass at 30hz i think. So its a low pass filter. SO when i have it at 30hz it will only play stuff under 30hz.