Storing your amp or preamp

Is there any harm in an amp or preamp being stored for two or three years without playing it? Assuming it is stored in a dust free room temperature environment.

Does solid state equipment kept in storage keep any better than tubes? Should you remove the tubes if the equipment is not being played for more than 6 months?

Either tube or solid state.The original box and a heavy mil plastic bag should do fine.I suggest wide plastic tape on box seams to keep out moisture and packing it up during low humidity day or use moisture absorbant pouch.Also remove any batteries(remotes,units with battery based microprocessor settings memory)so they won't leak over time.
Leave nothing loose in box that can roll around and damage.
I agree.
Just prevent it from extraneous dust exposure by placing it onto the plastic bag even if you don't have an original box.

Even if dust is accumulated within 2 years of storage it could be easily swept with soft paint brush when the box is open. I clean(sweep) the circuit boards on my equipment once in a while.
better to sell it now before 3 years of depriciation. You can buy the same model back in 3 years for hundreds less. If the co. comes out with the MK IV or new and improved, thousands less.
Be sure to let some air into the unit if you wrap it in plastic[condensation can form].Do not store it in a basement,or in a garage[moisture] Good luck,Bob
I seem to recall there were posts about this on Audioasylum stating that capacitors deteriorate if not used on a regular basis. However, I am not certain. Suggest that you do a search and post your question there.