op
first of all the somewhat imbalanced test readings on those tubes are nothing to worry about... in the mhdt dacs the tubes are in a buffer circuit at the output, so the tubes are run very easy and well within their comfortable operating limits, and so the minor transconductance (effective gain) differences across the two triodes at a narrow tube tester test frequency will be barely heard (if at all)
second, you should try the various tubes and see which you like best, i have found the we 2c51 adds more high end sparkle and air, which is a nice upgrade for some systems
third, in my experience (i have had 5 mdht dacs, including the stock and grannyring modded orchids) using a good 6sn7 or 6dj8 adapter opens up a much wider field of tube choices, my favorites are rca 6sn7’s (greater spaciousness and drive) and the amperex 6dj8 (same, with a nice spotlit midrange clarity)
the fun is in the tube rolling, so enjoy the learning experience!