How should a tube amp be stored ?
It should be placed on a shelf at my house where I can keep an eye on it, and hooked up to speakers to ensure the storage is properly working on a daily basis. (I practice ISO 9001 techniques.) If any of the tubes fail during storage I will immediately notify you so you can replace them to maintain proper storage conditions. Note: I do not charge for this service.
Ok, the answer you are probably looking for is that you need to protect it from moisture which can corrode the chassis and cause mildew to grow, and from dust which absorbs moisture and causes spots, corrosion, and other issues. The easiest way is to put electronic equipment (tubes or otherwise) in a thick-walled bag with a small amount of dry dessicant (silica gel, the same stuff used for cat litter) and then seal the bag, making it airtight. The thicker, heavier bags are less permeable to oxygen than the thinner ones. If you use one of the vacuum gadgets that evacuates the bag and then seals it, so much the better.
How long can a tube amp be stored before the parts start to degrade ?
Aside from the electrolytic caps going bad over time, which they'll do anyway over time, nothing else should deteriorate. You might get some minor outgassing from plastics, and oozing out of monomer in insulation, but that's about it.
Properly packed with a dessicant, your tube amp museum should be safe from the ravages of time for many years. It would, of course, be safer at my place where I can keep an eye on it, and where it would be covered by my homeowner's insurance. (Just sayin'.)