For the last 21 months I have owned a Yamaha RX-V663 in a 5.1 configuration without issue in "Straight Enhancer" configuration. I have a Tivo, Samsung Bluray and Xbox (the last two via an Oppo HDMI device) connected via HDMI.
Suddenly, without configuration change, whenever a 2 channel sources comes on TV (certain channels / Tivo sounds), Bluray (certain features in movies, startup sound on bluray) it now only comes through the two front speakers at insanely low volume. In order to make the audio sound bearable I have to now use some sort of Surround Mixer. Previously this was not required at all. It also seems to happen on ABC with secondary sounds, such as the show "No Ordinary Family" where the daughter is trying to read someone's mind or the background soundtrack. This happens no matter what setting I use. Another example would be penalties announced by the ref during NFL games.
Since then I have tried various things, made sure cables were all connected, reset configuration, re-ran YPAO (the auto setup Mic) but whenever I access a two channel source with Straight Enhancer, it sounds yucky.
I have made a video clip that captures what my phone picks up during tennis (regular multichannel) and when switching to a channel that has a 2ch audio. In the past this worked fine.http://dl.dropbox.com/u/271557/badsound.MOV
Support has no idea but they do admit something is wrong. I am very close to just giving up and buying a new receiver.