After looking at the manual pictures, I would say that it isn't really set up for using the XLR inputs as you would like. There are RCA preamp outs and amp ins that would normally be used if the preamp and amp sections are separated.
If you use the XLR inputs you will be going through the preamp stage as if your preamp were just another source device.
If you still want to do it that way, you would only need to use one of the pairs of XLR inputs and then select that input with your input selector.
Bill