trim pots next to 12AX7s on VTL TL-5.5

I have a VTL TL-5.5 circa 1999-2000

Next to the two 12ax7 sockets, there are to trim pots that are light blue on the outer perimeter and white on the inner part.

Any idea what these adjust and how to adjust them? Someone suggested to me that they were bias pots, but I've never heard of adjustable bias on a pre-amp. If they are bias, which legs of the 12ax7 would I be measuring and what values would I be looking for?

I can only think of 2 things they may be used for. The first is a gain control for an individual input. That would allow you to match the volume of more than one component so they are all the same. Second, if you have a built in phono stage, they may be adjustments related for that.
I'm voting for phono stage adjustment, but I'm not sure. This preamp originally had the phono, but the prior owner removed it for whatever reason. I haven't noticed any sonic degradation, but I just wanted to make sure my tube life was maximized by making sure everything was set properly.

I think I'll set them at the midpoint and forget about it.
"I think I'll set them at the midpoint and forget about it."

Before I did that, I would experiment a little, just to make sure they don't effect sound quality in some way.