The best thing you have going for you is the noise ordinance laws. They have to prove that you were indeed exceeding the noise ordinance IF there is one. I've heard about this alot around here and generally when people know what they're doing in court, they don't have to pay the fine. The problem becomes if they try and stick you with the infamous "court costs". This in itself can often be more than an ordinance fine, which often makes most unknowing kids just pay the fine before court.
It depends on how much power your really pushing when the judge asks, if he hears something like 1200 watts hes not going to be as lenient as 700 watts. The larger the power number the more asinine you sound for having that much power in a confined space.
At some point I'm wanting to replace the factory speakers with some quality Infiniti speakers pushing 700 watts. Check the link in my sig for info on car audio, good stuff there.
1. Find out the laws in your area, and the times they are in effect.
2. Read up on cases of similar citations people have had online and what their defense was.
3. The Constitution is your friend, use it any way you can without abusing it.
Usually cops will leave people alone with stereo systems, unless its rather bad. However there are the assholes that will write people a ticket if they can even slightly hear/feel the bass of a stereo system. Sometimes even just a respectable and professional call to the officers Desk Sergeant will stop problems you may have with newbie police abusing their badge.