That's why most of us have different valves to change the gain and impedance match accordingly, and why flyer also changed bias accordingly. And that's why for me after the speaker is Lampi, then the power amp to drive the speaker, and then the pre that will match the gain between the 242 and the rest. The flexibilty to change bias/adjust with valves is an advantage. And as for compares, they should be done across systems, like with cartridges
When you change the tubes and bias affecting gain by such magnitudes you are changing distortion patterns significantly - IMHO it is a different game, but as valid as any other in this hobby. Typical tube rolling uses electrical equivalent tubes and keeps gain, the main differences in sound are due to mechanical characteristics and noise spectra of the tube.