You'll find most car audio amplifiers are optimised for operation with a 4ohm load. Some amplifiers (usually class D mono-block subwoofer amps) are designed to run much lower resistance, even down to as little as 0.25ohm a channel.
So when talking about amplifiers go off the 4ohm rating and aim for subwoofer combinations that equal 4ohms because while amps may be able to run at 2ohms, they usually have better stats at 4ohms.
This is a generalisation, like any rule of thumb there are exceptions of course...
If you want to deliberately run lower, you'll need one of the afore mentioned specialise mono-blocks or similar multi-channel amplifier.