I am hoping that someone can suggest a ‘box’ that I haven’t discovered. I could use the receiver as an input switcher that integrates Bluetooth, etc., and ignore its internal amplifiers, exporting audio to external amplifiers, mixing the microphone along the way. Maybe there is a receiver that allows me to patch the front channels through a simple resistor network that mixes. This could be repurposing an eternal equalizer patch. In the old days this was fairly common on stereo receivers.