Comment Re:squeezebox family (Score 2, Informative) 438
Yup, the Squeezebox family of products is your best bet. It integrates fine with your existing setup (you just need a free aux input on your amp) and can be standalone (Standalone boom box). All of them support WiFi or Ethernet. You can operate each station completely independently or you can synchronize them (same music everywhere). If you have your musick already ripped to mp3 and your tags are clean then most of the work is done. The product family is about a decade old, so it has some history and the bugs are gone.
In addition the server software is open source and quite portable (Windows, Linux, even some NAS boxes are supported). There are plenty of plugins and extensions. Internet Radio is well integrated too.
It is not cheap, but none of the alternatives are cheaper or better either.
Markus