Grant,
under normal conditions, 6V filaments with a 6v supply, when the power is applied, there is an immediate surge of current that exceeds the warm operating current. With some tubes you can actually see the light from the heater flare up for a second and then settle down. This is the most stressful moment in the life of a filament. An analogous situation is the incandescent lamp. Light bulbs often "blow" when they are first turned on, because of this affect.
With the 12v supply and suitable resistor, what happens is the initial current surge is much less because of the resistor. The filaments will take slightly longer to warm up, but not by a noticeable amount. This affect is also known as current limiting.
I don't have any scientific data or studies to point to for this application but the analysis is correct and my observations with crude instruments support this explanation. One could hook up a time trace on a scope and record the peak waveform in both cases and I believe the data would tell the story.
As far as the rest of the operating conditions, the resistor solution seems to function quite well.
Fred
