Depends on how much power you have running....they are known to get very hot!! You have to dissapate a lot of energy (in the form of heat obviously) to get the power levels correct. I think the rule of thumb was that with 25-50W you can leave the covers on, from 50-75W you should elevate the covers at least 1" from the body of the crossover, and above 75W, you must take the covers off completely.

The bottom line, they have to have room to breathe. If this continues, call Matt at ID, or post over at the ID specific section of the sounddomain.com forum. Typically Eric, Fred, or one of the Team ID guys should get to it within a day or so.