Velodyne DD 18. It has some corection and perhaps your HT proc does too.
Would more subs be better? Depends. I like the idea of having one for the rear surrounds. One for the mains. Even though there is usually but one dedicated sub track.
Using an extra sub means extra stuff and blending it to which ever speakers. Lo pas. Hi pass. Cut offs. Etc.
Velodyne DDs can be daisy chained and give great results via the on board room correction.
Sorry, I can’t speak to the ML option. I’ve only had Velos.
Good luck.