Hi.... A lot of subwoofers contain their possess amplifier and a crossover filter before it. For these, a low-level preamp productivity is usually needed; feed it into the subwoofer amplifier's input and also into the main amplifier. For other subwoofers, which are set up for a speaker level input, just run them in parallel with your main speakers, or combine them into your system with your own bass amplifier and crossover? However, NEVER connect speaker level signals to an input which requires a low level (also called a 'line level') signal. You will cause damage, instantly when you turn the amplifier providing the speaker level amplifier on. Some A/V receivers have an electronic splitter specifically intended for use with subwoofers. If you have one of these, you will either want a separate amplifier for your subwoofer or an amplified subwoofer. Always consult the manual which comes with the subwoofer and the amplifier(s) you are using. Proceed with caution.