How do I correctly store 10 new preamps?

I came accross a handful of brand new 25 year old Phase linear preamps.
I sold a few and now I have to focus on work so I have to store them for a year. Is the attic ok? The cellar? Keep in mind that i have a 5 foot stack of them new in boxes so its not like I can just stick them in a corner of the living room.
Thanks,