I’ve been through nearly all the KT’s and still today, the best tube in that family has been the nos green label mil spec GE6550 in the white box, this was a honey of a tube, glass was twice as thick as any tube in that family, you could hear it when you flicked it with your finger nail it just made a dead thud, instead of a glass tinkle.
http://www.nosvacuumtubes.net/wp-content/uploads/2015/02/NOSGE6550A-1__764701.jpg
Cheers George
http://www.nosvacuumtubes.net/wp-content/uploads/2015/02/NOSGE6550A-1__764701.jpg
Cheers George