left speaker is louder than right speaker

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!