I have had that due to three things. 1) one channel was recorded or mixed louder, happened once on a poorly mixed re-issue LP. 2) The rectifier tube on my phono stage was going out and it resulted in a marked decrease in right channel volume, this occured only on LP's, normal sound on CD's. 3) most common problem, poor speaker placement, all the measurement were equal but my room and it's contents were not. I played a few mono recordings until the sound was centered and the soundstage was proper.
Now, that being said I am sure there are problems with upstream components that could cause this as well. My suggestion if the top doesn't help eliminate electronics upstream to down until you find the culprit.
Now, that being said I am sure there are problems with upstream components that could cause this as well. My suggestion if the top doesn't help eliminate electronics upstream to down until you find the culprit.