The amp had installed a couple RCA 6V6GTs. I don't know how to date these tubes as I see no code other than the "KX" shown in post # 11. How do you tell if the RCAs are "black plate" or "gray plate"? Are there tell tale characteristics? The glass is kinda smoked so its hard to tell for sure the color if thats the obvious way to tell. If they are black plates what are there value on the open market?