One speaker louder

One possibility is room acoustics relative to the placement of the speakers and your listening position. Window glass, differences in furniture upholstry, wall hangings, rugs, etc. can affect sound.

are you using tubes?????? If so its possible one channel is dying slowly.......Check your speaker cable fittings...Try switching speakers ( leave everything else the same )..Just a couple things..Hope they help...........

Assuming that you have indeed confirmed that it's the speaker and not something else, that is, you've swapped them left for right and the sound follows the speaker, then there are only a couple of possibilities:

1. Damaged speaker - a failure in the crossover network could easily cause what you describe, but occasionaly a driver failure will cause it as well.

2. Manufacturing tolerances - just because they're consecutive serial numbers or a supposedly "matched pair" doesn't mean they were perfect to begin with. Speakers are made by people, and people make mistakes.

i had the same problem for a while it just takes a lot of patients. it is all about first reflection and speaker placement