I got these Sylvania GT's last week.
The ones on either side are the ones you like, I believe.
What is the centre one? It has a full getter flashing but also a taller base. Could it be the same as the Syl 6SN7W? Alternatively, could it be the same as the tube I posted about last year (pictured below), which could possibly be the true Chrome Dome?
Have you tried listening to them yet?
The one in the middle is your jackpot IMO. What matters is the getter flash, notice how it goes so far down the glass it reaches the bottom mica plate? Those are the ones that sound great, I think they have shown up in both the normal and short base types. Sylvania mixed and matched on this stuff a lot. I have a pair with different bottom micas too, one has teeth and one does not but both tubes sound identical. The getter flash covering the whole tube is the only reliable indicator. There is a fine line between the GT and GTA, they can be hard to tell apart. One way to tell is that the flashing looks "cleaner" on the GTAs. These are the tubes I avoid, they don't sound as good. The flashing on yours looks a bit on the short side but I do still think they are GTs. They should all have 3 hole plates, I've never seen the 2-hole plate type with the heavy top getter flash in this glass envelope length. The 6SN7W is a top getter with pretty heavy flashing, and that one is 2-hole, but the glass is shorter and there should be a metal support rod between the plates. Your tubes are all medium glass.
Whenever somebody tries this tube and tells me they thought it was ordinary I have become convinced they're not actually finding the right tube. The real deal gives you that sort of "aha!" feeling of total transparency that very few tubes do.
I have a pair that looks exactly like the ones on the ends of your photo and I think they sound great too. What you primarily gain is more transparency trying to hunt down the rare oddballs. All three in the photo are great tubes and worth owning. I would try to hunt down a friend for the one in the middle if your amp uses pairs, it would probably be worth it. But remember we all have our own tastes too. Some people don't like the BGRP Tung Sol despite its reputation. You have to trust what your own ears tell you is good, not mine.
Edited by Xcalibur255 - 2/25/13 at 3:45pm