I can only repeat that the so-called "sound" of a valve is simply due to a spread in characteristics, unfortunately too prevalent these days.
The 'sound' of a particular amplifier has 90% to do with the circuitry, not the used valve. Very often (mostly?) comparative tests have been performed using only one or two valves of a brand; had the tester taken the trouble to test the characterisitcs of tested valves he was likely to have found differences in operating ponts related to the particular valve's characteritics, not the brands. (In those few hearing tests that have included measurements, different 'audible sensations' were always found to relate to valve - not brand - differences.)
I have used a number of 6SN7s over the years without trouble, but I have never bothered to compare between brands. Up-to-standard valves: predictable results.