Right channel louder, BAT VK-P10SE SuperPak with Ortofon 2M Bronze(MM)

I just received the used unit today :). I used the following setting:
I have used "user" (4 and 8 for S101) and "user" (4 and 8 for S201). S102 is set to "Direct" and S202 is set to "Direct". S2 is set to "Low".Now I notice that right channel is louder than left channel. I tried both Phase setting. What could be the problem?

Any help to diagnose the problem is much appreciated. 

Playing a different source does not exhibit this issue.

I had one of these.  Try swapping the tubes left to right, then front to back. 
My VK-P5 has a trim pot on each channel. I think the P10 has them as well.  They could have gotten set differently.  
@try1256 what is the trim pot? 
@dgarretson There are two rows of tubes and two in between the rows (somewhat). What exactly should I swap. I had to set S2 "high" for volume to be reasonable. Still the same problem.
@dgarretson I swapped the smaller tubes left to right and it sounds ok. Why did I have to swap to fix this?.

@geek101 Those swaps may obtain better matching of the gain sections, which tend to drift with time. It's also possible that by reseating the tubes you cleared some sludge and restored electrical contact.  For a unit that old, you should clean all tube pins and sockets with contact cleaner. Also, consider replacing at least the 6SN7 tubes. Those are in the power supply circuit and tend to wear out faster than the 6922 signal tubes.

The owner’s manual is on BAT’s website.

@dgarretson yes I did use the manual for adjusting the jumpers. I am going to clean them as you suggested. I did notice black stuff at the contact points.

I will look at replacing them, please suggest what would be good replacement for them.