For these boards the I2S bus is driven from the WM8004. This is because the RPi does not put out a master clock and we need a PLL to recover it. This means the inaccurate BCLK from RPi is multiplied to get a master clock. These clock errors create real problems for the downstream digital devices, causing loss of lock at random times. Because we are the master, a second board can not be added. There are XLR to RCA adapters that are reasonable priced. A quick search on amazon for "XLR RCA adapter" turned up quite a few choices.