upmpdcli (http://lesbonscomptes.com/upmpdcli/), the audio UPnP/DLNA MPD front-end now has good support for Linn Songcast, including synchronized multi-room setups.
Using Linn Songcast, you can capture the audio stream on a Windows or Mac OS X desktop and forward it to one or multiple players. This means that any audio-playing desktop application (e.g. Spotify, Tidal, etc.) can be forwarded to the (Raspberry Pi) Media Player(s), independantly of whatever proprietary protocol it may be using.
Instructions for setting this up here: http://lesbonscomptes.com/upmpdcli/sc2mpd.html