@tksteingraber Glad to hear that you are enjoying the NU 6F8Gs. They have a very special blend of sonic attributes that I find impossible to beat.
The 1982 vintage NOS Melz 6n8s/1578 tubes I bought recently are sort of a mixed bag sonically. Extremely quiet (none of the crackling I read about) with the best bass slam/texture/definition I have heard (I prefer their bass to that of my KR Vt-231s) and unequaled imaging/soundstaging. Transparency is on par with my Tung Sol VT-231s. Upper treble is smooth and refined. Unfortunately, they are very forward in the upper mids, thus fatiguing after just a short session. They have improved in this area with additional play, but so far the upper-mid forwardness largely remains.
There is a ton of discussion on the Melz over on head-fi.org. Members Paladin79, bcowen, and JKDJedi seem to be the foremost experts on Melz. IIRC they feel that the Holy Grail Melz 6n8s is the early/mid 1950’s non-hole shiny black plates with the same micas and metal supports as the later 1578s. Good luck finding those at this point!