Hi I have 3 of these stacked on top of each other. Is this okay to do? Will they overheat? The airvents are on the sides so I personally think they'll be okay but just wanted to check first. For info; one is used constantly (has music and movies on) and the other 2 are back ups so one is used every hr (Time machine) and the other one is accessed once per day for back up. Cheers in advance.