Regarding connecting a Line 6 StageSource speaker to a Powercab via L6 Link, the verbiage for the AES/EBU connection in the manual is as follows:
"When an AES signal is detected by the first Powercab unit in the chain, the chain will split the signal's left channel going to the first Powercab unit or Line 6 StageSource® speaker and the right channel to the second."
When using this configuration, the StageSource has to go before the Powercab. If the Powercab is before the StageSource, the StageSource will not play. We hope to address this in a future update.