Definitely don't mess with the fuses with the power on. It sounds to me like the problem may not be the amp at all, but rather the speaker you have attached to the right channel. I say this for two reasons. First, Rowland has had the amp twice and can't find anything wrong. Second, the problem exhibits itself when you hooked up the right channel in your last experiment.
I haven't read the entire thread, so I don't know exactly what you have tried, but here is what I suggest. Swap the speakers between channels. Hook up the left speaker to the right channel and see if the problem occurs. Alternatively, hook up the right speaker to the left channel and see if the problem moves to the left channel.
I haven't read the entire thread, so I don't know exactly what you have tried, but here is what I suggest. Swap the speakers between channels. Hook up the left speaker to the right channel and see if the problem occurs. Alternatively, hook up the right speaker to the left channel and see if the problem moves to the left channel.