That is exactly what I was trying to underline, and also that from my own experience I like digital amplifiers, which have surprisingly low prices with high power and sturdy stability when this property is critical for "difficult" speakers such as M/L.The conclusion that was made and easily provable was that SPEAKERS make the biggest difference in a system. In fact speakers dwarf every other part of your set up. Interconnects? Not much. Speaker wire? Not much. Fancy amplifier or preamp? not much. But speakers DWARF THEM ALL. I can go into more detail on why speakers dwarf everything in the sound system if anyone wants me to go into it. But, it just interesting info to know when you make decisions. The next issue that dwarfs everything is the room effects. The guys that do work on treating the room and its negative effects on the sound are way ahead of the game. You room treaters already know this......
I posted this as information NOT to start any flame wars. It's just something to think about.
Beyond that, anybody can make his own mind within his budget constraints.
But as you also pointed, one should NOT underestimate the technical difficulty of making unbiased amplifiers, loudspeakers or even cables A/B comparisons. For instance, when comparing loudspeakers, one should not only adjust the actual sound levels in the room (better than 0.5db), but also the position in the room of the loudspeakers, especially when comparing loudspeakers with different radiation patterns (like Martin Logan's vs B&W). And using the exact same position is also probably the wrong answer, since the "optimal" position will be different for the 2 kinds of loudspeakers.
Bottom line, as you said : one should be VERY careful when measuring/hearing subtle differences : only on LARGE differences (loudspeakers again...) can one be reasonably sure that no unexpected bias could explain most of the difference.
Note that this is not related only to the ears<->brain interface, but also to the eyes<->brain interface : the first time I saw Adelson's Chess Board, I could not believe my eyes, and had to use Photoshop to check that A and B squares were actually the same grey.