Sub for Olympica Nova III

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.

If a sub is sufficiently controllable and goes deep enough, there is no reason you can't match them to your speakers.  +1 for the Rhythmics

I used to own Sonus Faber Olympica 3 and now own Sonus Faber Amati Traditional. I have occasionally contemplated adding a subwoofer. I got rid of mine when I upgraded from Olympica to Amati. My biggest problem with my subs (2 B&W 800 series) was turning them down enough and feathering them in. I would not concentrate that much on the specs in buying a sub... I mean... max output, watts. What they will add is a very small sliver of the audio spectrum And what is important is feathering them in correctly. 

I would definitely add a Sonus Faber. While other speakers I have owned in the past have been incredibly ugly. The Sonus Faber sound incredible and look great. So, that is now something I actually think about. The Sonus Faber Subwoofers are made to work with Sonus Faber Speakers. Subwoofers, unless you are trying to shake your house with an unnatural exponentially excessive bass are going to be more than you need. Most subwoofers that I hear in people’s systems are turned up too loud and are screwing up the imaging and sonic presentation. 

I would unquestionably buy the Sonus Faber. 

I listen to mostly classical and jazz, and some rock and bluegrass. I have the Sonus faber Olympia Nova IIIs in a room that is 5m x 8m x 2.7m and use 2 Arendal 1723 1 Vented subwoofers. I'm very happy with how they blend with the Olympica Novas. Their app makes them very easy to control (loudness, crossover, roll-off slope, phase, multiplie equalization bands, option to control settings for both subs at the same time, etc). Free shipping and duty from Norway (and free return if you don't like them). Some considerations that led me to them were reviews (e.g., Audioholics both written and on YouTube, Nemo Propaganda on YouTube), and pricing (i.e., no cost for a showroom or middleman). But, boy, are they heavy!