LPF is largely personal preference, but generally for subs it will be somewhere near 80hz
Sub sonic filter depends on the type of enclosure. If it's sealed you can either disable it, or set it very low (~15). If it's ported it should be set somewhere close to the tuning frequency of the enclosure.
Bass boost - as close to zero as possible -- if your gain is set correctly you shouldn't need it.
Gain -- you SHOULD learn to set it with a DMM, it isn't that difficult