What change will give the most dramatic difference in sound? A rectifier swap or a power tube swap? How about the two small signal tubes?
I did it with the U70. Power tubes made a moderately big difference (EH 6ca7 was the most neutral, Mullards were very "warm" but they were also very old). Small signal tubes made a tiny bit of difference but I couldn't tell which was most accurate. Aside from the frequency of blowing up, I could tell no difference at all with the rectifier tubes, none, zero. I checked the rectified voltage and it changed the voltage a bit depending on which rectifier you used, but it was in the 1-2 volt range so it's essentially negligible compared to the wall power variation.
That was before I put in the series diodes, but I would expect that it would still make no difference, particularly since with the diodes in there, the rectifier tube is pretty much only there as a soft-start circuit.
JJ Rectifiers died the quickest, Sovtek close behind. I never got to blowing up a Mullard since it wasn't in there for more than about an afternoon. The Japanese/RCA that was in there when the amp was first built in 1962-3 still works but it well down on output voltage.
After the diodes went in, there have been no rectifier failures in use, and everybody has managed to remember not to power cycle it, which I expect would still cause a blown fuse and then failed rectifier, diodes notwithstanding.