NOOOOOO!!!!!!!
DO NOT DO IT!!!111
The output stage runs too much current for a 6sn7, even with both sections paralleled.
Its a cathode follower. The gain of the output stage is ~1 no mater what tubes you use, output impedance should (theoretically) be lower with 5998.
Im pretty sure the hum comes from the B+ which is not as well filtered as it is on the PPX3. PPX3 has larger resistors (more ohms) in its CRCRC supply.
From screwing around with my Extreme, I would swap the plate resistors in the gain stage with SS CCS's. The good news is it can PROBABLY be done without pulling the PCB. The bad news is my amp hisses and I dont know why (Its not oscilation, I just dont know where the noise starts). It may be that I have noisy cathode LED's (another mod I did to the amp) and I am too lazy to swap them back to resistors & caps to test that theory. Something to note if you do decide to do this is that the amp must NEVER be tube rolled outside of a certain family.