Weird rec tubes

[picnic]

I have an 07 LSS. I switched a Sovtek 5Y3 and put in a 60's CBS 5Y3. When I switch through the power levels with the CBS installed, I hear a distinct increase in the volume levels going from 5 to 15 to 30 watts. With the Sovtek, I do not hear an increase in volume going from 15 to 30 watts. There is an increase going from 5 to 15.
I know at 30/35 watts, the SS rectifier takes over.
Any ideas? Bad Sovtek?