Can I use the B1 with my original A5s?

Yes, you can still use the B1 with the original A5 model! With this kind of setup, you'd just need to use either an RCA to mini-stereo adapter (like the one that was included with your A5s) or an RCA to mini-stereo cable. You would just connect the RCA end into the RCA outputs of the B1 and the connect the mini-stereo side to either the top or back mini-stereo inputs of the A5s.