I'm confused as to where your DAC is. Do you have RCA or XLR cables running from your DAC to some other device, like a preamp? If the DAC is inside the preamp, are there RCA or XLR cables that connect the preamp to the amp?
If you have RCA cables involved, I would try to verify that both RCA cables are working. Hook up one RCA cable at a time so you can be sure that the cable itself works. Try one cable on left and right. Try the other cable on left and right. Doing a process like this can help you find out where the signal stops. You could even have a bad speaker wire. I've actually seen that happen before.
It's quite difficult to troubleshoot this with just words and no system diagram. I suspect that, because your computer with a different program (YouTube in a web browser) only plays on one channel, that you have a system issue and not a computer issue. It could be any of the components.
It might be helpful to have another source like a CD player or FM tuner etc.
Good luck with your troubleshooting.
Brian.