If you look at the back of your amplifier, for the leads that go to your speakers, there should be red and black terminals labelled L and R. With the amplifier powered off, take the cable out of the L connections and put it in the R connections, keeping the red and the black the same. And at the same time take the cable out of the R connections and put it in the L connections, again keeping the red and the black the same.
Then power up your system, listen to the same recording before. Has the channel inbalance swapped over?
For the source test, there should be an interconnect cable that goes from the source to the amplifier. There will be a pair of connections on the amplifier for this cable. One for the left channel and one for the right channel. With the amplifier powered off (very important) unplug the left channel input into the amplifier and put it in the right channel input and at the same time unplug the right channel input and put it in the left channel input.
Then power up your system, listen to the same recording before. Has the channel inbalance swapped over?
It's all simple logical trouble shooting. Changing one thing at a time, seeing if it's changed anything and taking things from there.
It's the sort of thing that if I popped round to your house, I'd expect to have identified the source of the issue within a few minutes.