Buzz from preamp

The left analog output from my processor is creating a buzz. I connect only main L/R to the amp and disconnect all other cables, including the processor's power cord. Floating the grounds on both processor and amp doesn't matter. Therefore, it is not a ground loop issue. Switching the cable from left analog output to any other analog output stops the buzz. Therefore, the buzz is coming from the Left analog output.

The amp is dc-coupled. Therefore, it seems like the cause of the buzz is a DC offset on the output of the processor. Perhaps the cause of the buzz is a shorted ac-coupling capacitor at the processor's output.

This is an older processor. In addition to the ac-coupler capacitor, I'll probably replace the electrolytic capacitors. But how about other capacitors? It's not practical to replace all capacitors. Does anyone have any input as to which capacitors are usually replaced in an upgrade/maintenance?
how old is it? it might be time to re-cap.
It is about 7 years old. I agree it's time to re-cap. Question is, which caps in addition to the electrolytics?
7 years old is far away from recap. It usually happens after 15...20 years. You may inspect cold or cracked solder joints. Coupling capacitors almost never go bad.
I think it would be a good idea to list your entire system. There's a chance the problem is not with the preamp.