Lower output stage coupling cap value and add grid stoppers.
Add cathode current sense for each 6L6.
Add grid stopper to V5a.
Add protection 1N4007 diodes to 5U4 anodes.
Change one input stage to cathode bias, and remove it's input coupling cap and add grid stoppers and 1Meg leak. Seto other stage grid leak for ok anode idle voltage.
Play and fault-find all creaks and groans.
Check hum level is not objectionable.
Check how much feedback is applied - may need to reduce feedback to ensure no instability for all load types.
Check which stages are overloading first, and maybe attenuate or increase stopper to move overload more to output stage.