Sir i would disagree. nails get loose pretty easily as there is nothing but the wood itself to hold it an whereas the threads on a screw give extra holding strength to it. the proper way to do it is to pre-drill the holes with a thind wood drill and then use 1.5" screws to hold the wood in place.
by the way what do u mean by steel packing under the wood? i used nuts and bolts to hold my woofer in my previous install and they held firm if you tighten properly. the reason these things become loose usually is that the carpenters here use half inch wood instead of .75" and the screw doesnt hold tight in that wood.