That cabinet is a dedicated subwoofer with a high end of 300Hz. All you would hear with it is very low bass, with no mids or highs. Plus, it's a very inefficient cabinet, meaning it would take a lot of power to drive it.
The mixer amp is a low powered PA amplifier. The 400 watts is a meaningless peak number, or what the amp draws. It is a 100 watt amplifier, and even that number may be peak watts, with the actual RMS power being around 50 watts.
It's not a good deal and definitely not a bass amplifier.