2x6, remember that rooms are the second most sound-altering component (right behind speakers) of the entire system. You have many amps in different rooms, correct?
Now, I'm sure over the years you've someone else swapped out different amps in the same room. Of these times, how often were you was someone else able to switch amps quickly -- as close to instantaneously as humanly possible? How many of these times did you ensure that levels were matched as closely as possible when you compared? Doing all of the aforementioned while you were in another room.
A fundamental principle of human sensory processing as that our senses have a poor memory. The brain does not store raw data. You can't re-access your unbiased impression. Once it's in memory, it's linked with emotion and bias -- e.g. "Whoah, that sounded crisp, clear, and wide open! I'm pretty sure I like it more than the other amp" is something your brain would file away for later retrieval.
So, how do we get around this limitation? Instantaneous, level-matched swapping! Even when done in a non-blind situation, it's a much more reliable method of discerning differences between devices (speakers, amps, media players, etc.). Of course, as all of us in the "brain trust" have tried to convince you of before, the only way you can definitively say that one sounds better than the other is to remove as much unwanted bias as possible. Unwanted bias comes from your eyes. It comes from knowing the price and build quality. It comes from everywhere but the actual audio you hear.
Which product sounds better to you is the only bias that matters. And, as has been shown time and time again, when tested in a rigorous scientific way, the theory that well-designed amps sound the same has not been contradicted. For this reason, it's as believable as the theory of gravity.
