Thank you so much for taking the time to write these insightful and well-meaning paragraphs!You really need a voltmeter. The changes you are looking for are so much smaller than a few dB. And your ear is so affected by volume and perception that you are lost before you started!
Consider first the fact that time and time again, experts from the subjective and the objective side of things routinely fail to hear the differences between amps. My favorite is the 1987 study involving Futterman, Levinson, etc. plus a pioneer receiver, see page 78 (but enjoy the rest of the old mag...):
The experiment was certainly level-matched. Nobody could tell the difference. I worked in a store that sold Levinson amps at the time, to be honest I wasn't surprised. This happens over and over, when it comes time to listening without peeking, nobody can tell. Even when they can, it is found that the amps are severely frequency response mismatched, or level mismatched, or not properly functioning. There is an AES article on it, it is behind a paywall since it is a journal publication, it is referenced here. Even Benchmark creates a FUD when they talk about crossover distortion, where they cite a severely broken amp to make their case. And, to be clear, Benchmark makes SOTA amp, no doubt they pair them with a SOTA marketing department.
And as far as electronic interactions, preamps seem way less likely to have a sound signature, unless they are broken or oddballs. And since many of our industry's esteemed experts (including staunch subjectivists who believe in all sorts odd and very expensive purchases) have already tried and failed to tell amps apart, hard to imagine subtle differences in preamps making a difference. I can tell you my SFL-3 only had a sonic signature when a tube was going bad. Aside from the volume control upgrade which addressed a noise issue, it was just a preamp. I couldn't tell the difference between the SFL-3 and a Bryston, Apt, Yamaha, or any other preamp for that matter, nor could anybody in their right mind.
You should also get a UMIK or other measurement microphone and get comfortable making measurements in your room. You will quickly learn that speaker placement is important, and the type of preamp just doesn't matter. Unless you use the PEQ on the RME, which is also makes a dramatic difference in the sound and how you can mange your room and the impact it has in changing the sound.
The UMIK-1 is on top of my shopping list, and it seems like I have just added a voltmeter. ;-)
Thanks you also for sharing the link to the blind test paper. I am by no means an expert in audio engineering, and these are all great opportunities to learn. I don't consider myself to be either an ideological objectivist or subjectivist. I'll see if a more accurate levelling of both signal paths makes it easier to confirm some of the findings from my ad-hoc listening session earlier today...