Doug, a suggestion about "Longevity" for 4-400's (Actually, any
thoriated-tungsten filament power tubes):

These tubes are very sensitive to filament voltage. Make sure the tube
pins and socket are nice and clean and shiny (don't use any
abrasives!).. Measure filament voltage at the socket. This is all to
make sure the tubes REALLY get the correct filament voltage to the
filament... It's 28 Amps for two tubes!

Tubes that are new or in good condition do NOT need the full rated
filament voltage for full output. But if you do not have good AC line
regulation at your location, it's hard to get it exactly right.

In Broadcast use, I modified some RCA and Collins transmitters by
adding a "Sola" type voltage regulator on the AC line, and an
adjustable autotransformer ("Variac" or "Powerstat") to feed the
filament transformer. With all new tubes, I could measure the
distortion percentage on the transmitted audio, and lower the filament
voltage until the distortion just started to creep up. And set a little
margin. As I recall, a 5.0V tube would initially run fine at 4.6 to
4.7V. On one transmitter, the tube life went from 4000 hours to over
9000 hours (a year, 24-7). Of course, the filament voltage was slowly
raised, and the tubes were 'retired' when the filament voltage for low
distortion got up above 5.0 volts. In noncommercial service, you could
get some retired 4-400a's to work for another 1000 hours or more by
raising the voltage to 5.1 or 5.2

-- Plenty of air
-- Clean bright tube pins and sockets
-- Minimum required filament voltage

were the main considerations.

Have fun!