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
