Hey Mike. IMO, I would rather start with one better sub, and immediately start budgeting for a second one. The 2000 series is a significant upgrade to the nsd and again, IMO two lesser subs won’t be as good as one better sub. Having said that, it would help a lot to know your room size and listening habits, like music vs movies, and how loud etc. duals can improve the experience, but it’s not plug n play, and while headroom is definitely a benefit, smooth response over multiple seats is the biggest advantage. If you have mostly one seat to worry about, a properly integrated single sub can be great.