A small mains to 6.3v transformer ought to be findable from somewhere, or Ed Dinning could wind one for you. If you think of a 4v to 6v converter transformer for a valve substitution, it still has to handle the full heater power of the 6L6, and so a mains powered transformer would need to run the same power. You could even use the primary of the main power transformer as an autotransformer to hitch a ride on the mains voltage adjustment.
Is there room to squeeze in an adaptor chassis with the 6L6 and heater transformer? Then the modification would be easily reversible if you ever came across an affordable PX4. If, say, a rumour started that their bass was slow and their midrange granular reducing both 'slam' and 'authority'
David