I have one and use it with my fretted JK and fretless Lobe. It really works great. The JK body has the pickup very close (like about half an inch) from the bridge. Because of that the signal from the pickup has a lot of high end. The Box is basically a low pass filter and it not only brings out the low end very nicely, but offers 5 really useful settings. The last setting rolls off so much treble that the treble setting on the amp has very little effect..... very bassy.
A very good description!