one speaker louder than the other

I am sure this has been discussed so many times but I could not find the specific thread..
One speaker plays louder than the other..what is the most common source of problems?
Can someone direct me on how to find the problem..
I play phono/vinyl/preamp/2 monoblocks/
Did you swap speaker sides to see if it follows the one speaker or if it is the room?
Hi Newbie,

Yes follows as I switch the IC from right to left..
This proves that my monoblocks are okey? and perhaps it is a preamp issue?
My preamp is also a tubed system.
thanks for any followup.
What is your source component (?) or does it with all sources?
Switch the small tubes, channel to channel, in your pre-amp. See if the volume reduction changes speakers.

If that doesn't make a difference, and especially if you have a critically aligned and triangulated set up, check your listening chair'a position to be sure some one hasn't moved it a few inches (I have to recheck mine every time the housekeepers leave). :-)
I swapped the 12AX7 of my preamp and was able to pinpoint the source of the problem..the tubes..
replaced with newer ones and now i am back to listening..