Darren,
Do the filaments need to run that hot? 
Granted, my experience with tubes is about 35 years in the past, but I only remember the cathodes glowing a warm red, even the radar magnetrons, not bright enough to read the newspaper by. I suspect that there is a reason, but it exacpes me.
Weston
Thorated fillaments, designed for such abuse, it excites the electrons better.
Talking of heat, the valve sits there at around 200 deg C, you could do toast with them. Certainly don't need the heating on.
Brill for the winter
