Balance control?

They are different from the loading plugs, which I don’t use with my Aphelion cartridge as per Keith’s suggestion. The pots are inside next to the tubes and are labeled, you just have to remove the top cover. You can turn them with a small screwdriver. On mine I ended up having to lower the louder channel a bit and increase the quieter one as the adjustment range isn’t huge. I’ll post a picture in my virtual systems section under "Final Set" so you can see them. They are the little blue pots. There are 3 per channel, gain, trim and bias. Don't mess with the bias pots. 

@bimmerman2 

Awesome Keith did that! That's a great mod. I wish more preamps with analog volume controls would provide a few high quality (high quality resistors) discrete steps of ~ 1/3rd dB in each direction L/R. That should provide enough flexibility for most use cases (more than 1 - 1.5 dB cumulative imbalance and something is wrong) without a significant SQ penalty. 

It’s pretty slick he has that inside there, the range of adjustment isn’t huge by any means, but it was enough to balance out the very slight imbalance I was getting. Took forever to track down what it was, did all the steps above, cart good, cables good, speakers/room placement symmetrical to the 1/16th of an inch. That simple little tweak of the pots brought it into perfect focus.

The trim pots in the Herron might alter plate resistance on the input voltage amplifier tubes, which would affect gain. In my opinion most audiophiles would get in trouble fiddling with such a parameter. Or they may operate in some other way. Regardless, Atmasphere said all that needs to be said, to help the OP. I hope he is following the advice already given.