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! :)
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.
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...