Hi,
One sub is plenty. The volume of car interiors is tiny compared to rooms in a house. It's been years since I've written on car audio, but I have a sealed-box (acoustic suspension) sub, about one cubic foot or maybe a bit larger, with a 10-inch driver (Pioneer) and it's excellent. It was driven by a separate 185-watt bridged mono amp, which got intermittent. So I bridged the sub output from the other stereo amp. Some car audio amplifiers let you do that.
It works fine, but I'm out of touch about newer car amps. These are 15 years old.
Regards,
Alan
