@nickwild thinking aloud you talk about 3 distinct zones in the garden. Functionally do you want to be able to control volume, content etc. Zone by zone ? If so why not 3 amps. No experience with the polks but they seem expensive. I used to use rocks in garden but moving towards Herdio Marine IP66 speakers. Just FYI as these are totally different animals to I think what you are looking at. Not advice just some info 
This stared as a bit of a fun project but ultimately I was impressed with th £25 per pair speakers for outdoor use. IP66 Nobby Box