Subwoofer to use with Dynaudio Xeo 20s and Bluesound Node 2i

I'm using the Xeo 20s wirelessly with the Node 2i.

I'm looking at options to add a sub.  Dynaudio make a few subs, in their home audio range the sub 3 and sub 6, and in the pro audio range the 9S and 18S.  I'm not sure if any of these are suitable.  Also the sub 3 and sub 6 are impossible to find for sale anywhere.  They're also way too expensive anyway.  The 9S is reasonably priced if this would work...

But I'mm confused on the connectivity.  Would I just connect the sub via the Node 2i sub output?  Or is there an option to connect them directly to the Xeos?  

Also I'm looking for a budget sub really.  Around $500, if anyone has some recommendations.  
Thanks