Now installed, you notice that the more you turn the preamp volume up, the more the tubes impart that sound on the signal. But you can’t play it loud.You will increase the load the tube preamp sees, as then you'll have two parallel loads to ground, and it "may" not be a very good impedance match.
So could you, theoretically, put attenuators (lets say -10db) between said preamp and the power amp to lower the output signal which you’d then turn the volume up and drive the tubes a little harder to impart more of the tube’s sound at lower levels?
Better off getting the gain reduced in the preamp by a competent tube tech.
Cheers George