Mike
Are you looking to use the CP800 as a DAC only for USB input sometimes and use CP800 volume control for USB input at other times?
If you go to the menu and select "source setup" Just select a source (I think you can set up 20)....name it what you want...like USB-DAC and "USB-MAIN". select the USB Back as the input for each of those sources....lastly....select the "pass through" box on the first source setup page for the USB DAC source...this will pass through the signal without volume control
and when you select the USB main input....where you did not check passthrough...you will have volume control....
To only use the passthrough signal for your second preamp....setup two different configuration settings one using the main xlr and a second using the aux xlr....you can then tie an input to an individual config....
Very flexible
Bill.....it looks like after further review...your strike call has be overturned