https://www.cnet.com/news/this-pint-size-168-audio-system-packs-a-wallop/ >The Dayton Audio DTA-2.1BT amplifier. >You might assume as I did the DTA-2.1BT is a two-channel stereo amp, but it's actually a three-channel component. The third ".1" channel is a subwoofer output designed for use with "passive," not powered subwoofers. That's fine, but passive subs are pretty rare, and since I didn't have one at the CNET office, I couldn't test the DTA-2.1BT with a sub.