A wrote:

Now, the second part of the question is: does it matter?

If the filament life under normal operating voltages exceeds 20,000
hours,
than dropping 5% and gaining an additional 18,000 hours, is probably
not going to result in a noticeable improvement in life. Forever is
forever.

FYI, FWIW...

Well, that is, kinda, why I was trying to bring the discussion FROM the
"manufacturers recommendations" (on large populations) TO what is
everyone noticing from their own (multiple) amplifier experiences and
what everyone else yaks about in terms of their own (multiple
experiences) with filament power tube lifetimes.

Yeah...20,000 hours is a whole lot of ragchewing. I think I never in my
life spent more than about 10-15 hours/week with amp filaments lit up.
Today, I have the 813 filaments on about 2 hours a week (and drive the
pair with a single 811 in gg and it's filament is on, also, and not not
voltage managed, either). And, I've never blown an 811 filament, ever,
and I had a few of those tubes in amplifiers, too.

FYI, FWIW

If we extend the universe of tubes observed to the TV and radio types, I
would have to say that it is very rare for me to find a burned out filament
in the 6.3 and 12V tubes. And very common in the very delicate 35-75V tubes
used in series string sets.

-Chuck