What are the best speakers to use with Home Assistant that have a good sound quality and a low idle power?
You need to define your use case here.
You need speakers that are always on and ready to play, like when you need a notification that someone is at the front door or if you can live with a little startup time.
Always on speakers require a lot more than a speaker that can be taken offline with a smart switch or maybe a builtin function.
And also, describe your demand on size and quality. there are speakers, and there are speakers. Playing hi-end music in your living room is way different from listening radio in the shower…
Sound quality should be comparable to Play 1/ HomePod.
I would like the versatility of the Sonos speakers with the idle power of the HomePod 
because the should be connected all the time or at least during the day.