Certainly sounds like you have a problem with the tube in that channel. Since the line stage only uses 1 tube per channel, try reversing the tubes and see if the problem then occurs in the right channel. If so, you've identified the source of the problem.
Tube Question
I recently installed an Air Tight ATC-1 preamp into my system. I noticed that the left channel was considerably noisier than the right but not so noisy that I couldn't live with it. Very recently, however, there is a low level, high pitched squealing sound from that left channel. Is this due to a bad tube? The line stage uses one 12ax7 for each channel. Thanks in advance for your help.
Jon
Jon