My buddy has two non-master Fender SFSR amps, one with NOS GT KT66's and the other with the TAD 6L6GC-STR. I like both but, prefer the one with KT66's for the "Beano" tone.
It's unfortunate KT 66's wont fit in a 70s Marshall combo ,although I have one with 7581a's in one of 'em