Heh, just passing through and see 'Vacuum Tubes' in a digitalist thread; suddenly it's 1960 again :-0
Well... you're talking about -literally- a 3 dB change in available average power ratio, especially at low frequencies [where most energy in the spectrum lies] depending on storage caps and PS 'stiffness', etc. - maybe a tad more than 3 dB difference for peaks.
You'd need the filaments on all the while, on the 'extra' output pair. But since the DC bias current through the output transformer windings affects its dynamic characteristics (ie B/H curve saturation) when it is "transforming", the power supply would have to regulate at both current levels too -- and, there's more.
ie, I think you're pining for a quite non-simple contrivance. Not clear what you think you're gaining - saving a pair of tubes? At the least, some load resistors at HV might have to be relay-switched, as would the plate leads for the spare-pair need to be opened/closed = sometimes under load* and without creating noise transients. Then there's the feedback loop, which must have correct values at both levels.
* which demands 'robust' relay contacts, which can suppress an arc.. and maybe some noise as they operate. the more I think of the consequences + how a tube amp operates:
I'd want $3K to make such a thing ~bulletproof, too - cheaper for a home-brew test box. Cheaper yet: leave them all on, all the time - and buy some spare output tubes for the next 100 years, for a lot less $. Your simple 'gain switch' could merely attenuate the input, at preamp/voltage level - no problem to contrive such a 'convenience switch', if you can find a push-pull pot/switch combo in today's Collector World [?] Prolly some of my Tek scope buddies could check their junk boxes.
(Also cool the suckers with a [big, slow ergo quiet] fan - something rarely thought of, even with high end amps in the golden era of Hi-Fi and of real engineers designing special xfmrs and stuff.)
My 3 kopeks.. ah then, nostalgia ain't what it -
moi
who recalls how shitty were the first 'solid state' audio 'components' -- and then the rise of Evangelical-Belief-audio with $2000 speaker cables. Barnum was .. well, you know. If I'd only saved a few Marantz Model 10s in factory boxes :(
PS - decidedly Lethal voltages/currents in these; some had >600V plate voltages with hundreds of mA capacity; enough joules to fibrillate even what passes for a heart in a Neoconman's sunken chest.