Yes, you can do this. You can configure any input on your CP-800 to be a "Pass-Through" type. See page 26 of the manual:
https://www.classeaudio.com/pdfs/Owners_manuals/CP-800_ENG.pdf
That way, you can use the left/right outputs of a processor (like SSP-300 or any other) and connect this to the "Pass-Through" input of the CP-800 preamp. It would allow you to use the SSP volume control as master volume for the home theater setup.