Thanks all for all your input. I think there are some good general principles here, as well as some specific suggestions.
What's really encouraging is the news that others have succeeded to intergrate everything happily enough.
We started out with the 'keep a loud house' approach, a fine principle indeed. However, six months down the line and with a little one that still doesn't sleep for too many hours on end, tiredness and mild desperation are pushing us slowly towards the 'whatever you do, don't wake it' school!
Ideally I guess hi-fi in the lounge is a keeper, and also get some other bits of kit to dot around the house for flexibility. How to do all this on the shoestring that has arisen from funds being redirected in to babydclothes and high chairs is a different matter! I like the sonos system, but obviously you've got to be able to afford multiple units before it really comes in to its own. that's where I thought the rCube might come in to its own, kind of like a Sonos unit that you could easily move from room to room.
As an aside, I also think that with all these kinds of docks and multiroom units, there is sometimes an issue over control. Although good old fashioned remotes, CDs, and buttons etc are now out of fashion, sometimes it's nice to just press a button and have some sound come out, without having to reach for a laptop interface or recharge a smartphone. When I think about having Sonos in the bedroom, I think about how much I don't like the idea of fumbling around on a touch screen on an iPhone to get the radio on at 6am. My old, now broken micro might have been tragically out of date, but at least you could just hit one button to get sound first thing in the morning... and of course, AA batteries in an IR remote lasted forever, saving hastle and promoting ease of access.
Cheers anyway.