The plan is to have a 20" sonotube .......
This is another area of concern. Using a 20" sonotube will leave you with a very narrow end cap on the sub end. Have you considered that aspect? That's why most of us use a 24" sonotube. I know you have space constraints.
Using an 18 Hz tune as mentioned above with a 4th order high pass at 16 Hz and 1200 watts will have a port velocity of 23.5 m/s. 1200 watts is the actual power a NX6000 makes per channel into 4 ohms.