I emailed TIC to ask their opinion on 20W per channel for the GS3's, and received the following response:
20W p.channel is acceptable although care should taken not to loose that power through “resistance down the line.” This is done by ensuring the use of the correct outdoor speaker cable gauge; 14/2 up to about 75’ and 12/2 near or beyond that distance.
So I think I'll try it with just the CAA66 at first - though it might be tough to get 12 gauge into those speaker connectors.
I went back to read the CAA66 manual, and realized I overlooked something important - Zones 1 & 2, which have both RCA line-level and regular amplified speaker outputs, also have a switch that toggles between "Fixed" and "Variable". I had previously assumed that in the fixed position, a signal was output to the RCA jacks only, and in the variable position, to the speaker connection only. Not so - that's what I get for assuming...
The CAA sends a signal to both the RCA & speakers, fixed/variable just affects what kind of signal. This makes things a bit easier, I hope - I could connect an external amp, leave the volume on the external amp at a suitable fixed position, and leave the switch set to variable output.
I'd really appreciate a recommendation for an amp - relatively inexpensive, only need one input, 2 channels at ~100W? Also the CAA has a 12v trigger out... do any basic amps have a trigger in so that the external amp would only be on when the CAA is on?