Over 5 years old;)
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I agree with the above that it is a noisy tube. You can pull the tubes clean the pins with 4-0 steel wool and try them again. If the preamp is over years old I would consider replacing them. The super low noise 6922 from here http://www.tubeaudiostore.com/tubes.html are the best you can buy. I use them in my ARC phone preamp (which I leave on all the time) They are at least 10 dB quieter than the tubes ARC used.
Some tube equipment companies’, "Standby" practices, keep the gear at half voltage and zero B+. That avoids thermal shock, at turn on. Sonic Frontiers doesn’t mention the half voltage thing. Still; keeping the filaments energized, is supposed to better for the tubes(etc), than a cold start. Not to mention: warm up, as regards presentation. You may have a tube, with an internal glitch, which corrects after fully heating. If the noise only occurs in one channel, try swapping the tubes between channels, one pair at a time, to locate the offender. If in both channels; there’s probably another issue. I’ve always given my tube gear a few minutes(each piece) to stabilize, before bringing anything else online.
It could be dirty 6922 tube pin/tube socket contacts in the Line One preamp.
Usually just pulling the tube/s out of the socket and reinstalling it repeatedly a few times will clean the tube pins and tube socket. You can also use rubbing alcohol sparingly on a q tip to clean the tube pins. Even then I would still install the tube, pull it out, and and install it repeatedly a few times to clean the tube socket contacts.
Please note that all the components involved are always in stand-by. The components involved are a Sonic Frontiers Line 1 preamp, and Parasound JC1 mono amps.Why do you leave the Line One preamp in standby 24/7? Even in standby you are shortening the life of the tubes. The tube filaments are still energized.