I'm about to test changing V4 and V6 tung-sols to Svetlana SED 12AX7. I have four of those brand new, I found the last supply of those tubes in local music store. They still have few left. Telefunken in V1 brings beautiful cleans with SED. Power amp tubes changed to better after I put those Svetlana's instead of original Mesas. I'm very pleased with my sound now. Sounds better to my ears than band mate's Triaxis/2:90 rack with same cabinet...