Tritton AX 720 Problem?

I recently purchased the Tritton AX 720 headset for the PS3, the first few days of using the headset were very good, but i now seem to have a problem involving the surround sound capabilities of the headset. I don't know if it is just me or if it is a problem with the headset, but i no longer seem to be hearing surround sound, but instead stereo sound. I keep alternating between stereo sound and surround sound - via the option on the amp - and i seem to hear no difference in the sound. The amp shows that the dolby digital signal is being recieved, but it still shows it even when i unplug the optical cable. I am very confused, and help and advice on how to change the sound back to surround would be much appreciated. Thanks.