Your confusing a -6dB sub-bass speaker with a subwoofer. Nothing wrong with that just be aware.
Californian Silicon Valley technology sold to a German company.
https://www.velodyneacoustics.com/en/
I can’t say if these folks are actually producing the DD Plus or their simply new old stock. I’ve been using these technology advances since 1983 and two DD Plus since 2011 without a problem. I see their price has gone up substantially for American purchase but your getting remote controlled, six customizable presets and total Auto EQ. The Swiss Army knife of subwoofers.
I’m not a fan of total system DSP so another model in there line up may be more appropriate and less expensive to your needs. Good luck with your search.