Your Yamaha has a pre-out/main-in feature, which is to allow something like a MiniDSP to be added. Since you connected the Emotiva to the pre-out, the Yamaha’s internal amp is no longer getting a signal. To fix this, simply buy RCA splitters (or mono male RCA to dual male RCA cables), so from the pre-out jack, one of the split signals goes to the Emotiva and the other goes back into the main-in port on the Yamaha.