I think Ken is on to something.
I did a quick google for the Kenwood amp and it has setup functionality that will play a test tone through each speaker to allow you to adjust the volume of each channel/speaker so it sounds right in your room. Once you know that the amp is working correctly, you can try using a "known good" DVD through your PC. If it works correctly, great, if not, check in the settings for your sound card to see if the output level for the channels are consistent.
Forgive me if I'm telling you things you already know but I have experienced settings in my av and pc setups getting changed without my knowledge (family or friends,etc.).