You could get an amp that has has speaker level inputs in between the Connect:Amp and your speakers.  Or you could get a 3rd party amp and replace the Connect:amp with a Port, which has RCA level outputs.  Or you could get the modern Sonos Amp, which has significantly more power than the old Connect:amp.   I use a Sonos Amp and have never felt that it didn’t have enough power for my 4 outdoor speakers, while the Connect:amp did feel a little lacking at times.

You might consider looking at more efficient (more sound per Watt) outdoor speakers too. Depending on what you have now and what you can get you might be happy with your current Sonos and have some speakers to sell off instead.