I don't know about how low the Crown DSP goes, I assume 20 hz. I would not tune a single 15" below about 18 hz or so, the reason being is increased excursion above tuning, and increased amounts of IMD distortion resulting from large diaprhagm displacements.
Also, with limited displacement down low ( cone area ) why would you bother with a slight sensory improvement down low at the expense of higher distortion above tuning?
If he killed the original tempest driver, he was WAY exceeding xmax, until the spider physically stretched tight. He either needs: to turn it down, get more cone area, and/or ensure that there is an appropriate filter installed.
Having a low cut filter at 20 with a final tune of 18-19 is not that much detrimental, it will definately protect the driver from excessive excursions down low, and you can also add a tad bit of boost down at 20 as well.
"You take the blue pill - the story ends, you wake up in your bed and believe whatever you want to believe. You take the red pill - you stay in Wonderland and I show you how deep the rabbit-hole goes."