Have you considered swapping the zone speaker outputs, as a temporary test, and powering zone 2 via the main outputs? See if the zone-2 out will drive the main speakers, also

Wait! Are you saying that turning on zone 2 will shut it down with NO speakers attached??? Maybe there's a wire strand on the terminals, or internally, or a damaged terminal is shorting to chassis.

Just some random thoughts. (I have a lot of them, but some aren't fit for print!)

Larry
www.fineelectricco.com