Please allow me to muddy the water a little more. I seem to have a mix of some of the stuff you guys have already posted. Now this is an extremely simple and fairly cheap option (at least for me it was). I inherited 2 of the
Bose Free Space 51 outdoor speakers with the house I bought a couple of years ago. These things are old (~10 years or so) but they were already wired into the house and they work perfectly (plus new ones start at around $500/pair) so I decided to use them. I ordered what appears to be the same amplifier that duckvader used in his system with the raspberrypi.
A Dayton Audio DTA-120 Class T Digital Mini Amplifier. This little amp ran me around $60 at the time, but sounds almost as good as the old Harman Kardon receiver that I hooked up on them just to have some music out there our first Summer in the home. It also is extremely loud. I have to limit the volume in the house at the amp or the kids will have our neighbors upset. Finally, I plugged in a $35 Chromecast Audio to the amp which is the only source I use for music out there. So for basically less than $100 I now can stream Pandora, Spotify, local radio stations through iHeart Radio, and even my teen-age daughters crappy music collection through Plex/Kodi apps. I did have to add an old wifi router bridge in our screen room which overlooks the pool, so that our phones had adequate access to the network. My house blocks wifi signals pretty bad.