These special 6SN7-GTB made by Hitachi in the 1960s, earned a permanent spot in my reference system after easily besting the "Holy Grail" of 6SN7 & variants, the Tung-Sol 6SN7 GT, Black glass, black plates, made in the 1940s! (it wasn't close; If it were a fight the ref would have stopped it)!
Note: the 1970's gray plates offer 90% or more of the black plates performance at a very attractive price. They beat everything else out there except for, their black plated cousins. You need very good - excellent hearing to tell the difference. Only, roughly half of the audiophiles could detect a difference in straight A / B comparisons.
One black plate quad $500, or $250 per pair.
REVISION: Only one gray plated pair left $150 per pair.