I have the Sony SRS-BTM30 speakers which I'm happy with but they are NLA from Sony and are getting harder to find. Sony has replaced them with something twice as expensive (~$200) that don't sound much better to me.
I've got an Apple TV and -- as you say -- it is very slick (check pout my blog about streaming videos through a "Henry" http://bit.ly/xFUK0U )
It's great for watching videos and boring a bunch of people with your holiday photos (grin), but not so good when you want to play music on the back deck...
Although it requires a power cord, a Sonos Play3 or Play5 would produce better sound than some cheap dock. At $300 and $400 each it is not necessarily that cheap but the lossless audio over Wi-fi helps and it is easy to carry from place to place. It does use it's own dedicated network so the $50 WAP from them is needed or you can plug in the speaker to a wired network connection. Any audio on the network is accesible with an app for remote control via phone or tablet.
No audio dropouts provided a decent enough RF coverage is available.
You can add devices to your hearts content. And no I don't work for them but know someone who has tested their stuff.
Whatever you get has to have great dynamic range. Just think of the Logical Song with that awesome base intro ba-da-da dumpa-dum-dum...and then the "who I ammm" outro that hits the stratosphere. Great song. And the speakers have to be good enough to make the itune sound like, er vinyl or at least as close as possible! :)