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?
If your preamp or power amp uses tubes, it could be that one of the tubes is weakening and reducing its output. Have you tried reversing tubes from each channel? It could be a phono cartridge going bad in one channel. Have you tried reversing interconnects upstream, such as those between your preamp and power amp, and between source components and your preamp? If you could isolate which piece it is, it would make it easier to fix the problem.
How long has it been this way? A lot of music is louder in the left channel. Does it do this with many different recordings?
The off centered sound seemed to just happen the other day. It was with any cd I played. my only source is a Meridian cd player. I felt I narrowed the situation to the preamp. I switched out the 6 tubes with another batch I had. The left channel still dominated. I plugged my headphones in the built in headphone jack and the headphones played balanced. I unplugged the headphones and the speakers are now center balanced. Not complaining, but what's up!
Maybe some corrosion in the headphone jack.
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.
Start at the source Bondmanp. If it occurs when playing CDs, try reversing the left and right outputs on it.