What subs do you think will pair up best?

Both are very good, both are very different. While you shouldn't actually be hearing your sub there is still a nature of presentation. The difference between one large cone and six small cones is quite substantial,

I can't speak to the Paradigm other than to say I owned two pairs of their speakers and in hind sight they sucked. On the other hand the Sub 1 looks to be a real power house.

Personally, I sold my DD18 for two DD12+. The two sub thing really works well in my room.

What you really need to find out is the difference between their EQ software from someone who has actually owned or sold both.

I kept my questions general because I wanted a full range of responses. (But, regardless of the forum involved, I get tired of the responses which begin with: “It depends…” or “I have not tried it, but… ”.) I already have an Infinity Intermezzo 1.2s. I have an opportunity to purchase a used Velodyne DD 18 (2007) for $1,700 or purchase a new Paradigm Sub 1 for about $2,200. This conundrum is a matter of opportunity.