timer sounds like it's your best option at this point.
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When I asked UHF magazine how to extend tube life they said the first thing to make sure of was that the tubes stayed as cool as possible. So make sure your amp has adequate ventilation and maybe invest in a silent-type fan.
You might want to try a heavy-duty timer that will shut off power to the amp after a preset interval. I have no idea about the possible consequences to sonics of such a unit, though.
And finally, a joke : aging may solve your problem ! Lots of guys over 50 have to get up and pee in the middle of the night. That's when I turn off my wife's bedroom system.