Speaker Amplifier Question

Yes, It's possible that the  power amp has a leaky cap or something similar. ALso, for the heck of it, try cleaning the internal connectors to the input of the power amp. A pencil eraser on the input jack connector can clean it up pretty well. Make sure it's unplugged if getting into the cabinet and you're not used to routing around live hot power supplies...etc.

You said you swapped the wires L&R and the buzz moved... OK... so, did you also swap the INPUT wires to see if the buzz is originating in the soundcard or somewhere else in the path?

Try disconnecting the INPUT all together and see if the buzz is still there.

A leaky cap would cause a hum, usually at 60hz. Depending where it is, it would also tend to inject hum into both channels.

If the inputs are disconnected, and the hum is still there and follows the amp channel.... then it's time to visit a good audio repair shop and have it looked at by a professional.

Note: a quick chat with a knowledgeable amp tech  will let you know, before they open it up, if it's worth the money to repair of if the money would be better spent replacing the speaker/amp with something newer.  They will make money either way you go so figure out YOUR best path through this.

You said you swapped the wires L&R and the buzz moved... OK... so, did you also swap the INPUT wires to see if the buzz is originating in the soundcard or somewhere else in the path?

 
+1 to Guitarhacker's statement above.
 
Although it could be the channel on the amp, you will first need to verify the amp is not just reproducing what it being sent to it. Troubleshoot the inputs as well, as you may have a problem before the amp.