I just acquired a pair of new Rythmik F15HP2 Subwoofers. I’ve only had them a day. Paired them with my Totem Element Metal V2, which I think share some pleasant similarities with your Sonus Faber.
I am very pleased with the various tools on the subs to accommodate dialing in the paring with my speakers. These subs are fast, detailed, articulate, and blend perfectly. They go down to 14Hz and will play very quietly as well as deafeningly loud. You might start with one sub, then add a second later, or go with two of the 12” subs now. Either route will be within or close to your stated price range.
And don’t forget to just enjoy the music along the way.