Wow, we've got a lot of people here who don't mess around! Just go straight for the best.
I plan on keeping my Melos a good long time. I myself have just procured a set of these to use as my back-up to the Siemens:
"Amperex 6922, PQ, white label, Holland (shield, gold pins) Slightly more forward midrange than the US version, above, pushing vocals a bit more forward in the mix. Otherwise, very similar. Another winner..."
Sadly, you've got to get these old tubes while you can. I decided to get my spare pair that would be the equal of the Siemens, hence the new Amperexes.
I'm all set!
As for the Siemens gold pins from early 60s-- they have about 20 hours of further burn-in. To the degree that it's possible for a tube to make a difference, these do. Some say swapping tubes in Melos does not make a big difference the way swapping tubes can really impact sound in other equipment. My experience is different-- I find the Melos very susceptible to enhancement via better tubes.
These tubes are just great in the Melos. They are more dynamic and bit edgier and brighter than the calm and cool Russian military 6922s. This is exactly why I chose them-- give the somewhat dry Melos a little "pizzazz". The Siemens are as they have been described. They have soundstage to burn, very big and 3D. Sound is full and detailed at the same time. They aren't especially lush or tube-y but more accurate and neutral. Very clear sound with excellent imaging and instrument separation.
Anyway, big thumbs on on this particular set of Siemens!