I had a similar experience with 6V vs 2V. This was a blind test inasmuch as it was completely unintended and so totally unconscious. T heard it directly, I didn’t because we started the session playing for me totally unfamiliar material. But it was (in the context of system changes at this level) night-and-day. We all vastly preferred 6V to 2V at the same listening levels (to your point) despite the fact that this was not the best theoretical match for the input sensitivity of my amp. So I am very firmly in the “6V camp” (at least in my current set-up). To those who have chosen output level based on specs all I can say is compare them with your own ears and then decide!