Is the B1 compatible with the original A5 speakers?

The B1 should still work great with the original A5s! You can use the B1 with the mini-stereo inputs on either the top or back of the speaker.  To use the B1 with the original A5s, you would need to use either an RCA to mini stereo adapter (such as the one that was included with your A5s) or an RCA to mini stereo cable. It's important to note that the RCA jacks on the back of the original A5s is an output and not an input.