yes you can on the 6L6 setting and bias them.
BUT, why would you want too?
with the MK-50 II you can EL-34 tubes. i'd choose SED EL-34's ahead of running the 66's.
for me with the MK II, the 66's would be something for if i'm really, really bored. the SED EL-34's are the Bee's Knees in the MK II.