XMD-3 question Subwoofer RCA jack

I have the Clarion XMD-3 head unit in my boat and a Clarion 10 inch sub in front of the driver's feet. The amp is a Boss C800 with 150 watts RMS per channel and the amp is run bridged for 300 Watts total - which should be adequate for this sub as that is near its max.

The stupid question of the day is this: The unit has a crossover that can automatically filter out the high pass down to either 60, 90, or 120 HZ. It can also be set on REAR to get full output. When I put it on any of the low frequencies, I get hardly any sound out of my sub. If I put it on REAR and then use the amp to adjust my crossover, it sounds like what I expect. What is the purpose of this feature and why does it seem to be so useless? Is it just part of having a cheap sub?

The adjustment is probably set low (the default is right in the middle). Check the owner's manual, you can adjust that level up much more. Can't be done through the remote, must be done at the head unit.

after you set it for 90 hz get out of the menu and hit the am button and it will say s.w. push the up button and it should turn it up.

I set it at 60, 90 or 120 and all 3 give almost equally poor sound.

Fruit cake,
The owners manual said default was 120.

If I use the crossover on the head unit, should I then put the amp in Full mode instead of low pass? It shouldn't make any difference, though, should it?

There is actually a level adjustment. I'll check mine for referrence.

Turn it on, and at the head unit, hit the A-M button until it gets to the S-VOLUME mode. I think it's factory default is 0, max setting is +6.