Wilson Sabrinas, electronics and Subs

I've ordered a pair of Sabrinas and I am wondering if I'll be needing my Rel sub? Also, any insight on how my Nagra MPA will sound with these new speakers? (Front end is a dCS Bartok)
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.
Nagra and Wilson are very synergistic. 
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
The Wilson/Rel dealer where I bought my main system and who displays Wilson Alexx and full Rel lineup told me even Alexx benefits by having Rel added. Subtle but definitely benefits. So, yes.