The base is perfect and also comes with a frequency adjustment knob which helps you to fine tune the base output from your system

I've placed it in the corner of a 12' X 15' den and the base is solid

Nice, tight, low-end base (not as low as some sub-woofers I've owned in the past).

so it was free, but based on the performance of this unit I would have gladly paid $400 or more for a good subwoofer

This subwoofer is powerful deep base

There is an old saying, "Location, location, location", and this applies to subwoofers as well as to real estate.