crossover

The gain makes the crossover more sensitive to input.

Technically it's not bad for the amp, but your sound quality is probably being degraded, since gain is sort of a pre-amplification

The way to set it up correctly is this:

Turn the gain DOWN or OFF in all devices...

Once that's done go to the head unit,   start turning the volume up, slowly, until you hear distortion,  turn the volume back down a bit until it disapears.

Go to the next device in the line (Probably your x-over)

turn the gain up until you hear distortion,  when you do, turn the gain back a little until it dissapears.

Go to the next device in the line (Probably your amp)

If it has a gain, repeat procedure...

Sounds better now huh?

Right...

Remember usually you don't want the higher frequecies reaching the sub, or the lower frequencies reaching the mids or highs.

My personal preference is to  have the sub play frequencies up to 90Hz,  and if i have mid/highs, cross those at 4500Hz,  but that's just my personal taste.

Subs shouldn't go over 120Hz not because you'll damage them or anything, but mainly because sound quality is going right out the window...

And mids shouldn't go below 80Hz (Unless you have really good speakers) because they're not usually capable of reproducing the low bass, creating distortion.