Is the slope you are talking about the slope of the subsonic filter, or are you using the SMS-1's low pass crossover?

Assuming for the moment that your talking about the subsonic filter (which makes the most sense considering your comment on how it is behaving), a lower slope will cause the signal to drop off more slowly below the filter point. This means that the lower slope means your SVS will continue to get some signal (albeit steadily more and more reduced as you more toward lower frequencies) below the subsonic filter point. A higher slope will cause the signal to drop off to nothing much faster.

With the Polks set to large, I'm wondering how you are getting any signal to the SVS during calibration. Large mains will mean that the processor won't produce a subwoofer signal from the SMS-1's EQ output.