GE/RCA redbase 5691 are very good tubes, rich, full, involving, but a little ragged, great for computer audio transports. Brimars 6SL7s and 6SN7s are the opposite, Sweet pretty clean smooth. The Tunsol 6SU7 are very very good indeed, a high grade 6SL7. Mullard ecc35 are good, in a clean you are there, stark, clinical, way. But better is the Mullard branded, Made in Holland ECC33, less gain and therefore more relaxed. Anyway I have them all here, ring me. As for 12At7, I have very tasty ones here, including the Hollands.