The 5881WXT is a 6l6 variant I think so can I swap them without biasing? Before someone asks "why don't you just buy 5881" or whatever, I already have the 6l6 tubes laying around so if I don't have to buy new tubes I don't want to.
Depending on the amp, you may need to rebias anyway. Most amps that are going to need to be biased need it no matter what power amp tubes you change to.
I've had it pounded in my head dozens of times that even if you're changing to similar tubes (In my case I was swapping out KT66s for 6L6s), you need to rebias.