Jautor, as usual, is absolutely correct.
I think you're getting confused with actually controlling the receiver and the separate issue of getting an audio stream to the device. Your original question asked about the former - using an app like Roomie, this can be achieved using IP, IR or serial. If the receiver is IP-enabled it can just hang off your network and your iPad can control it without the need for additional hardware (but this ONLY addresses receiver control functions). If the receiver isn't IP-enabled you have to use something like an iTach (wired or wireless) and control it using IR or RS232. You still then need to address the second half of the equation, getting a usable signal to the receiver - this is where the AppleTV comes in.
Does that make sense?