Hi Zac,
I take it the amplifier has no DAC and no optical inputs ?
If it does then I would connect optically to the amp from the B2.
If it does not I would use an external DAC with XLR outputs and connect the DAC to the B2 optically.
There are a lot of DACs around that will do this, depends on your budget
If you have a DAC with RCA outputs then you can buy RCA to XLR adaptors and over a short cable run I don't think there would be much perceived difference in audio quality but depends on your set up and what you are trying to achieve.
There are some very knowledgeable users on here so they may have some comment/direct experience to help guide you.
John