Hmmm.... Is the CP-800 new to you, or have you been using it a long time?
A drawback of the CP-800 (as you probably know) is the lack of sub level controls. You have to set the sub level(s) manually at the subs to match the mains.
On the CP-800 itself, I’d make sure that the Configuration was correct for the subs and output connectors you’re using and that the selected input is using that configuration.
Also, check the manual to make sure you’re using the right output connector for the sub. I believe that for a single sub, it’s the one all the way on the right, if you are facing the back of the unit.
Does any of that help?