please forgive me if im wrong.....
but
doesnt the dust cap help pull and push air across the voice coil gap wich is semi vital to help cool off the vc when the sub is playing.... ?
I imagine some subs are designed to work this way, and others are probably not
but - it seems like a bad idea to me to bump hard with missing dust caps as there would be far less air movment across the vc area to cool things down like this.