I’ve never seen an amplifier designed to take a signal from another amplifier output. Line level only in every case I’ve previously seen, but not an already amplified signal. 

Look at the back of your Connect:amp.  It not have RCA outputs like the Connect does.  It only has speaker level outputs.  So if you want to use an external amp with that, you would need to use an amp with speaker level inputs.  And I would run all your speakers off the external amp.  

Ah, you’re right.  Saw the RCA jacks on the back and stupidly assumed they’re “analog outs” as opposed to what they are, which in’s.

Guess the external amp can go back!

Perhaps just keep your eye out for a (cheap) S1 Connect:Amp/Zone Player on eBay (or similar outlet) and put two of your speakers on each and ‘group’ them in the Sonos App - it’s perhaps one thing to consider at least, without having to spend too much on the project.

Yeah, that’s the new plan.  I guess it’s a loss in revenue, but would be great if Sonos came out with a Connect that can manage up to X zones, vs having to have 1 connect per zone.  I would upgrade for sure to that vs continuing to try to find S1 connects to add into the system at $150 or so.

The speaker ports on the left side are inputs.  From what I can tell, it’s fairly common on multizone amps used for more wired setups.

Intersting. That must be an fascinating circuit board in order to handle the variable output from another amp and then send it out as another variable output, rather than a direct pass through.

I’m assuming the restrict in some way the amount of power that can be sent to that input? I can’t quite read the model number on that device to go look at the manual.