How audible it is would depend on the level settings for input and output. Not saying he falls in this group, but I know that a lot of people in this business have no clue about setting levels or, for that matter, what sounds good (never mind what sounds great). After working with some people, I don't assume any knowledge of signal path, level setting, knowing sound quality or how to configure a system in the best way. I have worked with people who couldn't calculate a series/parallel load if a gun was to their heads.

Personally, I doubt it's the amp and more a problem with the input level setting. Knowing that the output from the Sonos is 2 VRMS, the level MUST be lower than fully clockwise and in fact, the manual, which doesn't actually show the input sensitivity, recommends that the controls should remain at 12:00, as it comes from the factory. If the sensitivity when fully clockwise is 1VRMS for full output, I would guess that 12:00 requires 2VRMS.