OP you've basically just described a Sonos system using the 'Connect' Zoneplayer with local music systems
The 'Connect' is a pre-amp Zoneplayer that can be set with either fixed or variable line-out depending where you want volume control. You can have a Hi-Fi system in each room and the Connect converts each into a streaming music system linking them to a central music library (iTunes etc.) and providing access to on-line services such as Internet radio (TuneIN), Spotify etc. Each connect is independant so every room can do something different, simultaneously, but you also have e facility to link any combination of rooms for coordinated playback when entertaining.
What you need to get your head around is the system is a 'streaming' system not a distribution system in the tradition sence of multi-room system. The only central device required is a network hard drive with your music on it, that could be a NAS drive, PC or laptop. Content is streamed to Sonos zone players via IP rather than sources being selected and controlled. One thing you can't do is lock out any content, everything is available everywhere but as each zone is independant conflict is never an issue.
The Connect is £279 inc VAT and there is nothing on the market that does the same for that price, in fact not much for more either. Sorry if I sound a bit fanboy but I get very bored hearing of Rasberry Pi / Airport / whatever solutions touted as alternatives. Yes they provide access to music in rooms around a house but multi-room systems they are not - they lack so many features integral to Sonos it's laughable. They're just cheaper, that's all. If you can afford Sonos you're nuts not to buy it, it's really that simple.