@mulveling Has gave a very good base to prepare for a investigation.
A very experienced Valve user, gave myself the very layman advisory, the most Upstream Tube is the one that sends on the signal downstream, the more confidence there is in the quality of the signal at this stage will usually deliver significant SQ that can be built on by further investigation into tube permutations
I have carried out a fair quantity of 6SN7 Tube Comparisons.
I settled on the KR Black Glass VT 231, as the best attractor for myself.
I then learned of certain individuals making claims the Linlai E 6SN7 with approx' 100 Hours usage life has compared favourably to the KR VT 231. The reports being made on these Tubes are generally good and have got my attention.