I’ve ordered a pair of Sabrinas and I am wondering if I’ll be needing my Rel sub?
No. You'll be needing three more.
The Sabrina's are nice speakers. But, given their size, bass will be limited to their physical constraints.
For me, I would set them up without the sub and listen for a long while (they have to break in). Then, add the sub and see if you need it.- My gut feeling is 'yes'. In fact, adding a second would be even better. And, adding two more would be divine.
But, you don't mention how you are integrating the sub. Crossover, full range, etc. That will make a difference, too.
As far as the Nagra goes, it should have plenty of power, though I have never listened to one, I couldn't say how well it interacts with the Sabrina's compared to other amps.
I think that a property placed single sub between the mains will stun you once set up. It works for me exceptionally well with Salon 2’s , Kharma mini exquisite’s and Wilson Yvette’s. I place the SW near the right speaker and can tune it to seamless performance. I always run the mains full range. I think that multiple subs will create many issues of integration with the main subs. My SW is a B&W upper end sub with excellent room equalization. FWIW