I don't profess to be an expert, but from what i know, the answer is no, or at least not very safely for very long. nearly every Mesa that allows you to swap 6L6's for 6V6's has a tweed/full power switch, and Mesa constantly warns you that 6V6's must run in Tweed mode. I think they require less voltage than a 6l6 and running them at the same voltage that is desirable/optimal for a 6L6 will risk damage. I suppose in theory that running an amp in tweed with 6L6's and 6V6's, without damaging the 6V6's. I would just be concerned that the two amps have very different capacities and tendencies, so they are "pulling" on the system differently. Besides, i am not sure the sonic difference is dramatic enough between those two tubes to warrant the "risk".
when I switched from 6L6's to 6V6's in my Heartbreaker, Monsta-tone (who was working on the amp) had to rebias for the 6V6's specifically. So, unless you have very matched pairs to make up this odd quartet, you have that additional risk.
I know I wouldn't do it, but I can't give you a full technical reason, beyond that.
good luck,
Brent
