No head units that I am aware of have the option of directly driving a subwoofer. This would require a low pass crossover internal to the head unit. You do have the option of using a passive crossover to drive a dual voice coil sub or 2 single voice coil subs off of the head unit but since the head unit internal amp is limited to roughly 20 watts RMS per channel you wouldn't get much bass being limited to 40 watts.
The reason for that 20 watt figure:
Assuming 14.4 volts,
14.4 volts would become the peak, send that through the amp to become a sine wave and you would have (14.4x.707) 10.18 volts AC RMS.
10.18 squared then divided by 4 ohms is 25.9 watts RMS theoretical. Once you figure in the voltage drop of the wiring and the amp chip itself you can see why it would be limited to roughly 20.
For fun re-run the numbers starting with 12.0 volts which would assume engine off listening and you will see a striking drop in power output.
Car audio amplifiers put out more power than this by including a power supply to boost the voltage internally. Head units don't have the power supply because there isn't enough room to fit one in and if they did have one they would likely overheat.
The reason for that 20 watt figure:
Assuming 14.4 volts,
14.4 volts would become the peak, send that through the amp to become a sine wave and you would have (14.4x.707) 10.18 volts AC RMS.
10.18 squared then divided by 4 ohms is 25.9 watts RMS theoretical. Once you figure in the voltage drop of the wiring and the amp chip itself you can see why it would be limited to roughly 20.
For fun re-run the numbers starting with 12.0 volts which would assume engine off listening and you will see a striking drop in power output.
Car audio amplifiers put out more power than this by including a power supply to boost the voltage internally. Head units don't have the power supply because there isn't enough room to fit one in and if they did have one they would likely overheat.