Hi.
Naim is not necessarily bright, but some of the models tend to be, and the Nait 5i is one of them, Uniti 2 being another if poorly matched. Why not look at Arcam? I'm using an Arcam FMJ A19 + a pair of Focal Aria 906 and i simply love the combination. I prefered the A19 to the Nait 5si to say the least, and the 5i is even brighter then the 5si.
I think if you want a Naim that's smoother, the Nait XS (first gen) is one. Unitiqute is another i think (or the UQ2, can't remember). Arcam A19 and Roksan Kandy K2 might also be great matches for B&W685, also Rotel RA1062 or RA1520.
If you want Naim 5i or 5si, B&W CM5 or CM1 might be a good pairing (CM1 are small and don't go that low though), as they are smoother sounding speakers (they have some kind of top-end rolloff).