I live in an apartment, and my two 15" EONs are taking up a bunch of room. I have some covered, semi-insulated space outside that could accomodate them, but I'm worried that changes in temperature might create problems with the electronics. I live in California, where it can get pretty hot sometimes but never too cold (it rarely drops below freezing). Am I safe in doing this?
You should worry just as much about protecting your equipment from humidity. Humidity will affect the speaker cones, the electronics inside, and everything else too in more extreme cases.
Changes in temperature will cause expansion and contraction. Since different materials expand and contract at different rates, that can break down the cones and the electronics too.
Dust will affect the cones, connectors, and the general finish.
Finally bugs and spores may regard various parts of your speakers as sources of nutrition, a great place to put the next generation into this world, or a great place to call home.
These are, of course all worst case scenarios. But electronics tends to be expensive, so why take any chances? For these reasons, I store all electronics indoor at room temperature. If you must, here's what I recommend:
The silica gel will absorb the humidity that's trapped in the bag when you seal it--when the temperature drops, this humidity will condensate. And vaporize when it heats up again. And condensate... The packing peanuts will protect against shocks and trap air that acts as insulation to slow down temperature changes.