I have extensive transistor technician background, and I am a little new to tubes, but I will give this a try anyway.
It sounds like your concerned with the plate voltage on the push-pull output stage.
To determine if the response is linear, you need more than two points. Here you are comparing the effect 100% on the PT has on the plate with the effect 90% on the PT has on the plate. You only have two points, so you cannot determine if the response is linear or not.
If you are wondering if the response is the same; i.e. does a 10% drop on the PT cause a 10% drop on the plate?
I think the answer to that is no.
I think there are two things that work in opposite directions when you lower the PT voltage.
Lowering the PT voltage by 10% tends to
1. lower the plate voltage by 10%.
2. lower the screen grid voltage by 10% too.
2 tends to offset 1 a little; i.e. 2 reduces the gain of the stage which puts less demand on the power supply which in turn causes 1 to have less effect.
A guestimate is that lowering the PT voltage by 10% will lower the plate voltage by 7%.
The preamp and driver stages have not been considered yet. I think they would also tend to counteract 1 the same way 2 tends to counteract 1 and for the same reason; i.e. the preamp and drive stages plate voltages would drop which would reduce their gains. The lower gains would cause the output stage to not work as hard which would reduce the load on the power supply .
Including all of that, a guestimate is that lowering the PT voltage by 10% will lower the plate voltage by 5%.