The biggest advantage the Vandersteen’s may have is in the e.q. capabilities. Though having multiple subs helps reduce the need for e.q. Does the NAD have any e.q. capability?
Are you happy with the performance of the Rythmik you currently have? If you are, then adding more in the future should only improve on what you have now.
I agree that 80 his a good place to start with the crossover setting. If you go higher localization will start to set in so you would need to keep the sub(s) close to the main speakers but at 80 or lower you would be able to place the sub(s) in the most optimal locations in your room.