I've done a LOT of sonos with external amps, never had an issue with output levels. That said, if the audio is not audibly distorted, the amp should not be going into protect mode, no matter how "hot" the input signal is. My guess is that the speakers are dipping low in impedance and pulling too much current from the amp. The speakers are rated "8 Ohms (4.5 min @ 167Hz)"

I'd check the wiring impedance too, water in a conduit pipe can saturate the wire jacket and cause lots of bizzare issues.