It's good to see the majority is setting it correctly.
The rule of thumb is to start very low. Then bring it up until you hear the power amp compress. The tone will get darker and smoother. Increase or decrease from there to taste. Then adjust the output level accordingly.
+ 1 =)Unless I am not aware of the ability to do so already, it would be nice to have the option to tie the master volume of the amp to the output level volume of the amp block. That way you could change the master volume while the actual volume stays the same so you can hear what tonal changes are going on without having bias due to the Fletcher Munson Effect.
+ 1 =)