I had a wonderful experience several years ago, dealing directly with Mark
and Nick at Rogue Audio. With their guidance and thoughtful advice, I
swapped out all the tubes in my non-Magnum Cronus, and the sound of
music has been sublime. I replaced the original Svetlana EL-34's with a
beautiful matched quad of Genelex Gold Lion KT-77's.
The five small tubes were replaced as follows :
an RCA NOS 5814 to replace the EH 12AU7 gain stage tube, a pair of GE
NOS Five Star 5751 "goldpins" to replace the Sovtek 12AX7 input
tubes, and a pair of GE NOS 12AU7A's to replace the EH 12AU7 driver
tubes. Definite improvement !! I'm not sure how to describe it, other than
saying the music simply sounds better.