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.
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.
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
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.