No you are comparing different brands of the same tube. That is also what Keith is referring to when he says tube rolling preamp.... Mullard telefunken, 12ax or Au. Now imagine your VTL allowed you to change from.12ax7 to PX4, PX25, 101d, 300b (and of course different brands for each tube)... But different tubes, it does not let you do right now. And other dacs don't let you do anything
Swapping tubes just because they are pin compatible is possible, but most of the time changes significantly the technical specifications of the equipment, including distortion.
Again, my point is that simple measurements would clearly explain most of what happens when you swap tubes with very different gain.
BTW Luke Manley optimized his circuit around the 12AU7 - it is the tube I am using.