It depends on the subwoofer and amplifier. The symbol stands for ohms which is a measure of electrical resistance known as impedance. If the impedance of the replacement subwoofer is lower than the impedance an amplifier is rated to handle (i.e. the minimum stable impedance), then the answer is no. If it is equal or higher then you can use it, but the amp will produce less power at a higher impedance. For example, if the sub is 2 ohms and the amp is rated to 2 ohms, then you cannot connect 1 ohm sub. You could connect a 4 ohm sub although at 4 ohms the amp will produce about half the power. If the sub is 4 ohms and the amp can handle a 2 ohm load, then you can replace the 4 ohm sub with a 2 ohm model so long as it can handle the amplifier output at 2 ohms. If you halve the impedance, you double the power. In other words an amp that provides 200 watts at 4 ohms will produce 400 watts at 2 ohms, and 800 watts at 1 ohm, if it is stable at those lower impedance values.