What do you mean by barely turned up ? You can enjoy tube sound at any level.
It seems you have a lot of gain, are you using the balanced out of the dac going to the preamp? If so you could change to the RCAs, that will probably output less gain.
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.