You just need to adjust the bias. The trimmer's range should be wide enough for 6L6s.
I don't think that yoou will change the character of the amp, powertubes = nuances. But thats my personal opinion. If you want to change the character use different volume settings for master/channel vols, which works within a range . . .
I'd use only Svetlana 6L6GCs btw.