The AppleTV is for streaming content from your PC to your audio system. You would connect the AppleTV to your CAT pre-amp. Then, any content that you have, in Itunes, on your PC can be streamed wirelessly over WiFi to the AppleTV. Of course, this assumes that you have a wireless router in your home. It can also be hard wired to your router using Cat5 cable. Your computer has to be on when streaming content from iTunes. The AppleTV has no dedicated analog audio output so you either have to get a special cable, that breaks the audio out of the HDMI cable, or run it into a DAC via a toslink cable. Apple omitted the amalog outputs because most people will either hook it up to a digital input on an A/V receiver or run it into a DAC.
There are many different devices that allow streaming of audio and video. The advantage of the AppleTV is it ties in nice with itunes and also allows streaming/purchasing of movies from itunes. The downside is that the video content is limited to 720P.
I myself use a highly modified Squeezebox to stream music from a dedicated music server. A music or media server is a dedicated computer that does nothing but send content to your media streaming device whether it be an AppleTV, Logitech Squeezebox, Sonos or some other manufactures device.
Hope this helps.