first 2 answers are right, josh went into detail, there are specs. called dampening, if u start with 200 dampening (speaker control) on an amp designed for 4 ohm mono and give it a 2ohm mono load you loose control of the cone pushing and pulling and weaken power handling,
i have seen cones moving all over the place and give low spl. and that can damage them.
if you stray out of the best specs say 2 ohm or 8 ohm on an amp that is designed for a 4ohm load you will either loose half the dampening, or 8 ohm. you will loose half the power.
each amp is best suited for a specific range, like a car, some are fast, some have better gas mileage, some have more towing capabilities, you cannot get a sports car to tow a heavy load, you can't get a tow truck to do 120mph easily, and u cannot expect an economy car to do much more then get good gas mileage.
8 ohm is rarely used in car audio but your situation there is a huge gap 4 or 6 ohm, 8 ohm will give you weak bass, 2 ohm may be a bit beyond your amps specs,
i would surely go for the 2 ohms, cause it is likely closer to what you need.
in the future give your subs and amps model # and ask this question again but 8 ohms is uncommon unless you add another pair to get 4 ohms.