I believe you want to utilize the volume control (preamp) of the preamp along with the input selector and use the power amp to actually drive the speakers. You can do that only of the Integrated amp has pre-outs. What are the exact devices in your setup?
See if your current IA has Pre Out. Usually most IA’s have Line Level Out and hooking up a PA using Line Out is not a great idea. You will have no control over the volume unless you can find a PA which has Singe volume control for all channels or has a remote to control the volume easily.
Line out has a fixed voltage signal unaffected by the volume control.
Pre-out is normally a variable signal since it is intended to be connected to a power amp, in this case adjusting the volume control adjusts the output signal level as a power amp is traditionally fixed gain.