When a variac is used to reduce the 120 V.A.C. going into a tube guitar amp, this not only reduces the high voltage A.C. , which will later become high voltage D.C. , but it also reduces the low voltage A.C. , which is used for the heater filaments. A better solution is to have an onboard user adjustable, high voltage D.C. adjustment control or controls, to dial down the high voltage D.C. on the preamp, power amp, or both.

 

Why overthink it? the purpose of dialing down the voltage is to alter the sound in a creative way.

All that work, and it might not sound good at all.

 

A better plan is to get a Digital-Class-D-Amplifier and derive all of your
Tone-Effects from a LapTop-Computer running Emulation-Software.

Then eliminate all the "inside-the-Guitar" Tone and Volume-Controls,
( except for Pickup-Switching ),
and replace them with a Wireless-Stereo-Pre-Amp.

The Pickups may be standard or active.
Active Pickups have zero-Noise and are much less fussy.
Single-Coils and Humbuckers can both be emulated in Software with the click of a Mouse.

And, replace your Speaker-Cabinet with 2 top-notch PA-Speakers on Stands,
or do a Direct-Feed into the Main-Mixer,
for the killer Stereo-Effects that You will now have at your disposal.

Do not use Stage-Monitors, use Ear-Phones instead.

All Pedals will still work with this type of setup.

A Midi-Foot-Switch-Board can be a very useful addition.

Once You experience this type of setup you'll be yelling "why didn't somebody tell me about this !!!!

There are NO Genres of Music or style that will not be seriously enhanced by going this route.
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When a variac is used to reduce the 120 V.A.C. going into a tube guitar amp, this not only reduces the high voltage A.C. , which will later become high voltage D.C. , but it also reduces the low voltage A.C. , which is used for the heater filaments. A better solution is to have an onboard user adjustable, high voltage D.C. adjustment control or controls, to dial down the high voltage D.C. on the preamp, power amp, or both.

is this a statement or a question?
Increasing the heater voltage will reduce life.
So, will decreasing it will increase life?
Yes, but not if the valve runs out of space charge for the cathode current that it needs to conduct. Then it will reduce life. (Of course, that applies to output valves, as preamp valves are rarely run at the maximum cathode current.
So, run them at the voltages that they were designed to run at.