This new old stock-testing RCA 6680 / 12AU7A / ECC82 dual triode was tested on our Paul Hart-calibrated TV7/U for shorts, gas and transconductance. There were no shorts or gas.
Transconductance measured a nicely-balanced 92/95 where minimum (65%) is 56/56 and 100% is 87/87.
The 6680 is the industrial version of the 12AU7A. It has gray 12MM plates; excellent flashing for an extremely-hard vacuum and extended tube life and a top-O getter.
I have played it in my Bottlehead headphone amplifier and it sounds outstanding. It is dead quiet when the volume is muted and has excellent frequency response, sound-staging, and dynamics.