wirring the subs in series raises the impedance. Say if your subs are 4 ohms a piece... wirring a pair in series will give an 8 ohm load... effectively cutting the amplifier power in half. An 8 ohm load is higher than car amplifiers are rated at... this will let the amp run much cooler and draw less power, but like I said, the subs are only getting half the amps output power.

What impedance are the subs?