Wilson Sabrinas, electronics and Subs

stereo5, yes, I would imagine they're very expensive. The suggestion was not to buy them but simply to get a sonic baseline.

I'd suggest someone purchasing new Wilson speakers should shop for a subwoofer with a lower frequency response that could act as a master controller for more economically priced array units to add on as needed. Equilaziation and output connectivity could be confirmed by their user manuals online. 

As a result of my in home experience with the $8900. Studio III and the new company's reticence for any significant innovation to the original British design other than cosmetics, the only endorsement I could offer is that any sub/s is better than none at all. 

A Wilson sub would cost thousands of dollars more than a REL sub. I think a pair of REL to start, then later on if you want more add a couple more for a swarm. You don’t need a very overpriced Wilson sub. 

In my in home comparative experience REL are not subwoofers. They are self described as a "Sub-Bass-System" because of their early rolling off.

Their speaker (high) level connectivity is just their old school output-less receiver connectivity remarketed by the new owners as amplifier matching BS. The idea is to match the speakers not their amplifier. REL does this by using a list of sub to speaker pairings. WTF?

Speaker matching is best done with detailed equalization and all the signal from a preamplifiers outputs. A REL may add a couple of sluggish Hz to those Sabrina’s but that’s it. Do your homework. Read subwoofer manuals setup instructions (not the positioning) compared to REL’s.

If your Wilson dealer doesn’t have a Wilson subwoofer on hand find a real Wilson dealer who does. You need a sonic baseline not shinny chrome.

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.

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

Nagra and Wilson are very synergistic. 

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.
Bob

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.

Subs, EQ and good room treatment are always a good thing, but see what you think. :)