I have a similar issue with my used C-J PV11 preamp. I will try mono, and switching around cables. No headphone jack, so I can't try that fix. I suppose there could be issues with other components in your preamp (caps, op-amps, etc.). In my situation, I think that the asymetrical layout is causing an acoustic imbalance between channels.
left speaker is louder than right speaker
The volume on my two channel system is off center. When I reverse cables, the volume reverses to the other speaker. When I press mono on my Sonic Frontier pre the sound does become centered. I can manually lower the left channels volume but prefer a fix. Any suggestions?