Sonic Frontier Line 2 Preamp Noise

I have been using a Sonic Frontier Line 2 Preamp for almost a year (bought new from Upscale Audio) and it sounded great until recently. Currently, there are noise coming out of the left chanel and I have narrow the problems down to the preamp. Does any body know what causes this problem? I did replaced the stock tubes with NOS recommended by Kevin of Upscale Audio. Could it be a bad tube? Any help is greatly appreciated. Thanks.
I had problems with intermittent noise with my Line 1. Turned out to be a bad tube. I would definitely check that out first before sending it in for service. Good Luck.
Dear Coolsound, Suggestion: Interchange the tubes. If th problem is then in the right channel.... you've found the cause !!! tube...or tubes. Let us know. cordially, Richard
Thank you very much for the replies. The problem was with the tubes. I replaced the NOS tubes with the stock tubes that came with the preamp and the problem is now gone. I can't believe the NOS tubes went bad that quickly (less than a year). It must have been a bad batch. Again, thanks for the help.