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?

Thanks!
Mark