Not a huge fan of black base gz34 , they seem too aggressive and hard at times. Also not liquid like the metal base version.
I would definitely agree with what you say about the black base GZ34 (although I've never heard a metal base), but I do like the forwardness/aggressiveness of the tube. Besides if I want a liquid/fuller/bigger sound, I use the S/P 274B, so its nice too have the different tubes depending on the genre/headphone I'm listening too, or the mood I'm in. Plus those metal based tubes are going for $800.00+, and I paid $860.00 for the WA6, so I just can't see it…plus I'm more than satisfied with the the amp/tubes that I have, and need too listen to them, and stop buying tubes for a while, which I've already said before, but…you know how that goes.