So I have this headunit in my boat, and on page 12 of the manual ( it clearly shows that there should be an option to set the REAR to SUB.

When I go through my menu it does not show any option for REAR.

Could it be because I am trying to set these settings while connected via AUX and should try it with a CD in the unit?

I tried on tuner settings and still did not find the option.