There are several factors to consider when buying a subwoofer. Here are some important points to consider:

1. size of the subwoofer: the size of the subwoofer depends on the size of the room in which it will be used. A larger room usually needs a larger subwoofer to produce a balanced sound image. Smaller subwoofers may be adequate in smaller rooms.

2. power: The subwoofer's power is measured in watts and indicates how much energy it can produce. A higher wattage usually means a stronger bass.

3. frequency range: The frequency range indicates which frequencies the subwoofer can reproduce. A subwoofer with a wider frequency range can reproduce more bass detail.

4. connection options: Consider how you want to connect the subwoofer. Some subwoofers have RCA connectors, while others need to be connected via speaker cables. Make sure the subwoofer has the right connections for your system.

5. enclosure type: Subwoofers can be made in different types of enclosures, such as sealed, bass-reflex and bandpass. Each type has its own advantages and disadvantages, and the choice depends on your personal preferences.

By considering these factors, you can ensure that you choose the right subwoofer for your sound system that will give you a powerful and balanced bass experience.