I would just store them. When you do plan to use them, it would be best to slowly bring them back to life using a variac (this allows the capacitors to slowly "reform"). If you hear noise, particularly hum, after the amps are up and running, it is most likely some power supply electrolytic capacitors have dried out and need replacement; any competent technician can do that, and if the amps are valuable, that would be a small price to pay.
I suppose "long-term" i s not really a precise term, but, I've heard many amps that were not used for more than 30 years that work perfectly today, even without capacitor changes. A friend recently bought an amp that is probably 65 years old that is in perfect working order and sounds great; it has all original parts, but I don't think that he knows how long it had been stored before he bought it (I would guess quite a bit of time because the tubes are ancient).
I suppose "long-term" i s not really a precise term, but, I've heard many amps that were not used for more than 30 years that work perfectly today, even without capacitor changes. A friend recently bought an amp that is probably 65 years old that is in perfect working order and sounds great; it has all original parts, but I don't think that he knows how long it had been stored before he bought it (I would guess quite a bit of time because the tubes are ancient).