My #1 tip for SPL
Don't treat your vibration damping as an afterthought. To get the most out of your damper, you need to install it beofre you build your box and before you figure outthe resonant frequency of your car.
Most people build the box and tune according to the resonanat frequecy, THEN add the sound deadening once they realize how much vibration and unwated flex they have. In some cases adding vibration mat to an already tuned system can lower your score.
In some cases the score might go up but mostly, this is just luck.
Losing a few points when deadening is added is rarely the fault of the vibration damper, and usually the fault lies in the lack of priority that was placed upon the damper.
By treating the vibrating sheet metal with the sound deadener first, then builing and tuning your box according to the newly altered resonance frequecy, you insure that you will not lose any deebeez, and will get the most out of your system.
That my tip for SPL.
ANT