I noticed that when connecting a Sonos PORT to an outboard amplifier that the volume limit on Fixed output will work while the player is playing audio, but will reset itself to OFF or 100% after the player is stopped and started once again. This is not the correct way to set the volume limit setting when using third party amplification and volume controls.
ONLY Using the Pass-through setting, keeps your Volume Limit setting in place thus allowing you to maintain the desired gain output of the Sonos PORT to your outboard amplifier system and not overload the inputs to the outboard gear.
I thought this would be very useful info to the right people, as I had chased this issue for a while not knowing if my third party control system (Savant) was causing this to happen or if it was a software issue of some other sort.