wrap the sub in a paper bag, then place it in plastic not completely sealed to keep moisture/rust levels down.and keep dust off.I have always covered mine with towels, and placed them in an unused room closet, and make my own shelves, and kept doors shut
If they have no real sentimental value to you, I honestly recommend selling them and just buying new when the time comes. Tech changes so fast, its VERY likely that you will be able to purchase something superior at a decent price when the time comes. (that said, I have tons of audio sitting around, unused, so i understand if you wanna keep them for sentimental reasons, in which case, audiobaun has a good idea)