Sub for Olympica Nova III

I'm sure that your are going to hear this repeatedly, but you really do want two subs.  You can always start with one and add on as budget permits.

Can you share some details about the space where they will be playing?  Subs work and integrate best when they fully pressurize the room with low bass so the room really matters.  

If your budget will allow, I would consider the new Cinnamon Malabar VLF sub from Portugal.  It is one case where you will not need two subs (and I agree that you usually do).  This one sub connects directly to the L and R of your amp's output, using the same signal as your speakers.  It is highly controllable and built to high standards. This relatively small, single unit can sit between your speakers.

 https://cinnamonaudio.com/loudspeakers/malabar-vlf

They are custom-built to order.  I had mine wrapped in a wood grain that closely matches my speakers.  To purchase, reach out to Colin King of Gestalt Audio.  You can find Gestalt online and just give Colin a call for a full breakdown on this excellent sub.

the Gravis is a bad subwoofer because of how limited it is for the asking price. such a laughable price

Get this - https://www.jtrspeakers.com/jtr-captivator-rs1

Order by sending request here -  [email protected]

Thanks for all the feedback so far. Appreciate it 

Space is a 6.76 x 6.05m open plan lounge, kitchen and dining (double volume, sloping ceiling). Listening area within this space is approx 3.5 x 4 with sofa 2.5 -3m from the speakers. 
 

I can get a Gravis III preowned for half the price of new one here in Australia. What draws me to it is the voicing being same as Nova (one would assume) plus the ability to setup and calibrate via app. 
 

+1 on Rhythmic.  I have a pair and they are fantastic.   So many controls, I can mate them well to a variety of speakers.