Octal Rocket tubes

[JoeCon]

Guys,

I've been running KT66's in the RJs Rockster (octal Rocket) I built and they sound great. But like all tinkerers I'm wondering about other tubes in the same circuit. Surely El34's will work but could 6V6's as well?
I'm seeing 383 on the plates and 28 volts across a 200 ohm cathode reisistor for the pair. Perhaps a bigger resistor would be needed for the 6V6s? Vintage RCA ones, made for others, can be had for about $50 a pair.

Thanks...