The Ultimax 18 may be able to take the power of the iNuke6000, but if that amplifier is rated accurately at all, the Ultimax would not be able to take that kind of power for long. It would cook the driver. Even the iNuke3000 should have enough juice to cook the driver, but it should be more difficult. I would use the iNuke6000 to power two Ultimax 18s.
Regarding data-bass graphs, do not use the basic response graphs to compare output. That graph does not measure output, it only measures the shape of the frequency response at nominal levels. To compare maximum output, use either the CEA-2010 Max Burst graph of the Maximum Long Term Output.