Reylon- Great tube choices! However, I don't believe that Sylvania ever made the metal base VT-231. These were 6SN7A or 6SN7W metal base of the JAN-CHS- variety. Both the Tung-Sol and the KenRad VT-231 dark glass are excellent tubes as well; and though a little hard to find, are certainly less expensive than the metal base Sylv.
