Just about all the modern receivers, pioneer etc, support airplay. So you can get as loud as you want with the typical receiver and speaker system. I have a pioneer VSX-1123, tall main speakers, and a HSU sub... now that is loud and clean even in a larger room.
a lot of the A/V receivers have it built in. just need a network connection of some kind either wired ethernet or wifi
or... you can get an airport express and add any powered speakers of your choice.
it's easy to expand the system, if you want to add music in another room, just add a device in that room. You can stream to them all in sync. (this only works from iTunes. From an iOS device you can only stream to one set of speakers at a time) You can also stream whole system audio from your mac, but only to one set of speakers at a time.
if the music you're playing isn't in iTunes, look at airfoil it lets you send audio from one program of your choice, or whole system audio to multiple locations at a time.