I am lucky enough to own what i consider to be a cracking good Marshall re issue of this amp. The stock 5881's are fine to me but with the re emergence of KT66 tubes everywhere thanks to the VM do i get the option to change the tubes like for like?? The 5881 is basically a 6L6 and i believe the KT66 will fit straight in. Obviously the amp may have to be re biased, but i also cannot see anywhere on the net how to re bias this particular re issue??? I do rebiase my TSL, VM and Fender Twin (sorry!) so i am not without some technical ability. My JTM re issue is also the larger cabinet 2x12 so the previous reported space problems are not apparent. I would appreciate some advice !!! Regards Rob

Well, I can say that I know it can be done.
Many years ago I had a bluesbreaker RI that the previous owner had put KT66s in. Not sure what all was involved in the changeover though.

As far as biasing, the reissue is just like the original in that it doesn't have a knob to adjust the bias inside.  You would have to change resistors or put in your own knob.