Hi Dave, the need is for a dc return, nothing in the way of capacitors will work.
Using AC on the filament a center tap for the dc return will null hum voltage.
The concept was raised of using a small filament transformer with a center tap having around say a 3A secondary at a voltage the same or close to the tube's fil voltage.
What hasn't been noted yet is that the primary shouldn't be powered up on the added transformer. that would lead to high circulating current in it and overheating.
Reid
Brandon DX