The transformer is probably not regulated so it might be 12V when it is playing loud and 15V at rest. The car voltage will be close enough.
The transformer probably limits the power if the speakers are turned up too loud. A car battery is extremely powerful so turning up the speakers might blow up the amplifiers or the speakers.
Connect a resistor in series with the positive DC power wire. 0.47 ohms/3W will drop the voltage 1V when the current is 2.1A. The 13.8V will drop to 12.8V and the total power in the speakers will be 27W.
Then the resistor heats with 2.1W, the speakers play with 15W and the amplifiers heat with 12W.
Are you going to bring them indoors each night?