Puzzling preamp problem

On some, but not all sources (CDs, records) the right channel of my system seems to overpower the left. This mainly happens in orchestral music.
What’s  puzzling is that most sources usually sound balanced between the two channels. When there is a singer or soloist the sound comes from the center. But when loud instruments (brass mainly) are in the right channel, the left channel seems to migrate to the center.
But when there’s a particular designated voice or instrument in the left channel, it’s very audible in the left speaker.  Very puzzling!
I assume it’s a preamp problem. I have a vintage Conrad-Johnson PV 11 preamp.
The situation has arisen with a variety of amplifiers so I don’t think the problem is there.
Has anyone else experienced this phenomenon?
I guess I’ll have to break down and get the preamp looked at.
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Since it is a tube preamp, maybe one of the tubes is at its' end.
I would call CJ, I bet they know the answer.
I  ordered a new set of tubes yesterday.
That is a possible solution.
You could also just swap tube channels and see if the issue moves to the other channel.
I have a reverse channels knob on my preamp.  When I reversed the channels the right channel still predominated.
Yet...... On many CDs the balance is perfect!?!?
It just seems if the right channel is very strong, it wipes out the left.
When you reversed channels I suspect all you did was reverse the source output, not the pre-amps out put. Swapping the tubes would be more revealing I think.
Could this be normal? I listen to a lot orchestra and experience this phenomena all the time. With an orchestra arrangement, the 1st and 2nd violinist sit up front and to the left of center (stage right) and thus are often louder. When there is a soloist, the soloist is typically front and center or just to the right of center.  
Swop the cables over connecting the pre- to your monoblocks.  What happens then?
I’ll try swapping the cables and see what happens.
Swapping the tubes is more of a hassle because I have to remove all the tiny screws from the chassis and remove the cover, but it’s worth a try

Interestingly though,  I had a several hour listening session with a friend today, and after everything was warmed up, the problem disappeared and everything sounded great.
So It appears to be a warmup problem, which leads me to suspect the anomaly is caused by the preamp, perhaps a weak tube.

Swap the IC's to know for sure.

With an orchestra arrangement, the 1st and 2nd violinist sit up front and to the left of center (stage right) and thus are often louder.

Sometimes true. But when the brass section hits, the mid right and right side is louder.

Well, the problem seems to have resolved itself!
I just had my monoblock amplifiers returned to me, modified last week.
They seem to have broken in during my long listening session yesterday, with the result that the whole system opened up, and lo and behold, the channels equalized.
I know I mentioned the problem occurred with other amps, but it wasn’t as acute.  Anyway, for now, I’m a happy camper!
Thanks for all your suggestions.
That's weird. I wouldn't think breaking in would affect balance.
I still think you might have a tube on the edge of going bad.
But, I hope I am wrong.

You could be right.  
I Had another thought:
Since there are two amps, possibly one of them got broken in before the other.
But, who knows?
Channel gain is not affected by break-in. It can be affected by being broken though :)