[morzh]

Putting 400VDC between the grounded filament and the cathode of a rectifier tube is really not a great idea. The insulation will eventually break down and short. Its just a matter of time.

Yep first time I saw that, even before I knew of 6X5 problem, I thought that was not a good thinking behind that idea.
Direct heater rectifiers did not have that problem.
The only thing was, they were usually high filament current types. Like 80 with its 2A. Or 5U4 with its 3A.