the Sylvania is the 6201. it replaces the 6AT7. as for the 6922 replacement, get the russian one. that's the one i have. gives the sound a much fuller body. but as markl has said, the 6201 is the one that makes the most difference.
i am also wondering if the caps inside can be changed to oil fiiled caps. i opened it once. doesn't look too difficult. that will be my next tweak.