call andy at vintage tube svcs (no i do not work for him). he has always given me good advice and tubes. and if i do not like them....he will take them back! i am buying tunsol 6550s from him for my rogue 120s. i have heard electro harmonix has a good kt88 (per mark obrien at rogue). rogue also sells kt88 selects with thier name on them that go in their magnum up grades. i would buck up and go nos tubes. the difference in my equipment (pre-phono pre- amps; tuner etc) have been big. i am sure there are other good nos tube dealers out their too.
good luck
good luck