The curve can be steep, but if you can get past it the basic operation is pretty straightforward.

So your mains are reproducing a signal all the way down to 20Hz? With speakers set to small, receiver set to stereo but not any sort of bypass mode (stereo bypass would cause the receiver to skip bass management), SMS-1 crossover off, and with the SMS-1 muted (or the sub off) you should see the response drop off obviously at 80Hz. I don't know much about your particular receiver, but I'd suggest checking that it is not in any sort of "bypass" mode.