Do you have any other source that you can try? If you have the same issue when using another pair of speakers, and you've ruled out the pre-amp, that only leaves the source (Oppo) or the amp. If you have a tuner, receiver with pre-outs, iPod or some other unit with RCA line outs, you could try it and rule out the source and/or isolate the issue to the amp. If you try another source and don't hear the problem, you know it was the previous source (Oppo). If you still have the problem, then it would seem to be the amp.
TIC