Almost sounds like pro logic has been set when playing dvds. See if, when the amp is set to dvd, check what dolby mode is in use. Play a dvd while doing this.
Look to see if the rear speaker audio has been turned down, there should be individual settings.
The usual, check wire connections.
I have a 5.1 set up, Sony micro speaks and a Sony Receiver. I also have a Toshiba DVD progressive player. I had this set up for almost a year, and now it seems that when I play a DVD, I get no sound from the rear L/R surround speakers. Sounds only come from the front speakers, the center speaker, and bass from the subwoofer. I used the test tone on the receiver and I hear the individual test sounds coming through each speaker correctly. Also, on regular TV, I heard sounds coming from all speakers. I doubt it is the DVDs in that they all generally have a 5.1 setup. Checking the audio set up on the receiver, it shows it as bitstream vs PCM. Has anyone experience anything similiar and any suggestions?