This applies to the Liquid Channel
In terms of data content there should be no problem connecting between AES and S/PDIF ports. The problem with connecting them arises from signal level issues. An AES signal amplitude is 5V whereas S/PDIF is 0.5V. Therefore if you use a correctly wired XLR to RCA phono connector to connect an AES output to a S/PDIF input, the input will be receiving 10 times the signal level it expects. We advise contacting the manufacturer of your interface and checking with them if any damage will be caused in the long term if an AES signal is fed to its S/PDIF input.
When considering a return connection from the interface S/PDIF out to the Liquid Channel AES input, it is the case that the AES input will receive a signal 10 times less than what it expects. In this case the S/PDIF signal level may not be high enough for the AES output and errors could occur, although if it does work then there will be no harm done by using a RCA to XLR cable.
The correct wiring for the XLR to RCA cables you should use is:
Pin 1 of XLR to cable shield
Pin 2 of XLR to pin of RCA connector
Pin 3 of XLR to shield of RCA connector
Wiring a 110 Ohm resistor between pin 1 and pin 2 of the XLR will act as protection from burning out the SPDIF input on your sound card.
Note that you can purchase converter boxes to convert AES signals to S/PDIF level and vice versa, although these will not provide any noticeable improvement in sound quality over a cable.