Ive heard the BX5s and the bass is ok, but a subs bass would be an improvement for the lower frequencies. I would say that the BR5s like the BX5s probably arent playing the lower frequencies very well so augmenting them with a capable sub would be the best path. I would go 2.1, and then when the time comes, I would highly consider surrounds next and a center channel after that. I used to run a 4.1 setup, and the stereo imaging was just as good with movies as with music, but with the benefit of surround sound for movies or blu-ray concerts.
Just a different suggestion.
Dynaudio Focus 260s, Focus 210C, DM 2/6, Hsu VTF3 MK5 HP, Oppo BDP-103, NaimUniti 2, Naim Stageline N, Pro-ject 2xperience Classic, Marantz SR7007, 60" 4K Samsung LED LCD