Here in Yuma there are radical differences in the volume levels of different agencies. It doesn't vary from mobil to base as much as from repeater to repeater.
I'm currently experimenting with the "light bulb in series with the external speaker" trick. As the current to the speaker goes up on loud signals, the impedance of the light bulb goes up (when it starts to light dimly) and the speaker increase is reduced. It has helped considerably on the most loud stations. I still need to obtain and try some other voltage/wattage bulbs, and will report the optimum one when I find it.
George