Everything seemed to be working fine. I had my PC's Creative Labs Audigy sound card connected to the rear stereo mini jack and I would play a bluetooth receiver and a CD player by swapping cables on the front inputs. The majority of the time I listen to music through the PC. Here's where the weirdness comes in. When I play streamed music on the PC, i.e. Pandora, everything works fine. However, if I play MP3s or CDs through the PC, after about an hour of playing fine, the music gets real distorted. Sometimes when I pause the music for 20 - 30 secs and restart it everything will be okay. One time I didn't pause the music because I wanted to see if it would clear up on its own. It didn't - in fact, after a minute or so the rectifier tube starting glowing a bright orange. I was afraid the tube would get so hot the glass would shatter so I immediately turned it off. Amazingly, the rectifier tube doesn't appear to be damaged as it is working as it did before.
The rear jack is connected to the front switch with a shielded cable (Mogami W2947) with the shield grounded on both ends. I have not had any audible hum on this input. Again, everything, including the PC, plays fine through the front input and streamed music plays fine through the rear input. I connected a different PC to the rear input and it seems to play okay so the problem appears to be some incompatibility between the Creative Labs sound card and the amp. I've been using the Creative Labs board for years with my Denon solid state amp and have never had a problem.
My theory is that when playing back higher "quality" music (I consider MP3 and CD to be higher "quality" than streamed), the Creative Labs board generates more harmonics and these harmonics aren't being suppressed enough by the shielded cable and then cause a problem in the amp. I'm not sure if I should be trying to do more shielding on the cable or if there is some mod I can do on the amp to prevent the problem. I'm hoping someone on this forum can provide some insight.