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.
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 )
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!