You're fine.
I generally turn everything except the preamp Im using up to 100%. right now I'm using an integrated amp turned up to 100%. This takes the variable resistor that is the volume control pretty much out of the circuit.
The fact that you only need to turn your preamp up just a little says you have more power than you need. Most people do.
You're still getting full potential of the tube amp to imprint it's sound. 100% of the signal is going through the tube circuit.
Jerry