Upgrade for 70s vintage Svetlana el34

I like Svetlana's el34, atleast those from the 70s, but can anyone suggest a tube that would be a noticeable improvement? Also,if possible, please tell me in what areas the tube you suggest might be an improvement. Thanks very much.
I have compared the Svetlana pentodes to the vintage RCA and GE and found Svetlana to be the winner! If you find a NOS tube to beat it - it won't be across the board.......just in certain areas!


Why not try the Cryo'd Svetlana el34 from Tube Depot? $ 150 a quad . cheaper than a nos quad and may sound as good and last longer ....I haven't yet tried it but will eventually....( Thor monos )