In one room I use a regular large stereo amp. I have a IR led taped to it to turn it on and off via LIRC because it uses 100 watts on standby and I can't stomach running it all night. It gets audio via snapcast through a raspberry pi 3.

In another room I have my home workstation/server/NAS hooked to nice speakers via a $10 mini Kinter MA170 amp that I thought for sure would sound like crap but honestly is pretty darn good.