One Channel out on my Sonic Frontiers Line 1 pre-Amp

Switch the tubes as suggested. If that identifies the problem you simply buy a couple of tubes on line. Buy new production tubes - they are relatively cheap and will work fine in your pre-amp. There are many sources (previously discussed and recently on this forum) and might cost from $15 to $40 or so). If that doesn't work have a local techie look at it. 

You could flip the tubes from the one channel to the other to see if the other side drops out but I think I would just stop where you are first before something else goes. 

Guessing you've checked all cable connections and etc. See if you can find a tech locally or at least someone with a tube tester. 

Plenty of Line 1 users on here so hopefully someone can give you more direction, but I'd stop where you are because likely it's an easy fix. Could try getting ahold of Chris Johnson at Parts Connexion as well.