Ok, I see by reading your post again, your cats are real stubborn jumping on the amp when its in use. You still may be able to find some pattern of putting decorations at the edges of the amps that still allow the vent to be unobstructed. If the top of the amp is above kitty eye level they probably won't try to jump on it believing theres no where to land.
If that fails just let the cats have their way and find some type of nylon mesh that will allow the amp to vent but keep the cat hair out.
If that fails just let the cats have their way and find some type of nylon mesh that will allow the amp to vent but keep the cat hair out.