I looked up the city ordinance there in Lawrence, MA but nothing was available. Finding out exactly what the ordinance you were cited for says is the first step you should research before fighting it. Here is an example from CA law:

27007 VC: No driver of a vehicle shall operate, or permit the operation of, any sound amplification system which can be heard outside the vehicle from 50 or more feet when the vehicle is being operated upon a highway, unless that system is being operated to request assistance or warn of a hazardous situation.

Based on reading this law, you can't have your stereo playing loud enough to be heard from more than 50 feet away. If you think about how far 50 feet is, that's just a little more than 3 average car lengths bumper to bumper. No decibel meter is required, just good judgment from the officer. For example, the officer hears music blasting from your car from across a large intersection and the officer is able to articulate that in court, you will probably lose if the officer can prove beyond a reasonable doubt that it was coming from your car.

Good luck fighting your ticket. I'm sure the cop was an a**hole only because you got a ticket and you thought you were singled out. Sounds like what the cop was doing was within the law based on your story.