If anything a semi-sealed box under the hood is a better environment as long as the amp has a fan to cool it.

Allow me to quote from the owners manual for my a/d/s P-850 amplifier in the "mounting locations" section:

ENGINE COMPARTMENT MOUNTING
Don't even think about it. The PowerPlate (tm) was not designed to endure the harsh chemical and heat environment of the engine compartment. Failure to obey this warning will void your warranty.

Well, DO think about it. The under-hood air temperature in a modern car can exceed 300 degrees F. There are lots of chemicals in close proximity (anti-freeze, gasoline, lubricants, etc.) and despite the best efforts of the EPA, some of the more volatile constituents will escape into the under-hood environment.

You, as a human being, can actually survive a more absusive environment than your amplifier. Would you be willing to spend 10 hours in a box inside your engine compartment with a fan conveniently placed to blow 300 degree air onto you?

tanstaafl.