Depending on how the plug in meters are designed, I'd think there's an adjustment you need to make to compensate for the additional resistance added by the meter circuit. That was a real head scratcher here when I was adding meters to my custom build. I ended up adding trim pots to the meters. I still have test points to use a VOM, set the bias that way, the adjust the trim pots to get the same reading on each tube.
That's a one shot process for initial setup. As mentioned, all you're doing is calibrating the add on circuit to match the amp.
I'd assUme that'd be easy enough to do with what you're using if it's a simple plug 'n pray installation IF there's an adjustment on the meter modules. Can't really get more specific without knowing exactly what you're using and how it does what it does.
PS, when I say meter error, I meant meter error ... it was what some might call ... significant.
(And ya, I know I got the VOM on the wrong range ... so shoot me already)