A-link (PCM / I2S direct) specifications
As opposed to the S5/S5b, which are regularly equipped with A-link input, the AYA 5 was originally somewhat DIY oriented, and its A-link input is only optional.
Moreover, the AYA 5 input switch is limited to two physical sources. In the default setup, one source is its USB stage, and the other is its S/PDIF stage (sporting BNC and Toslink input connections). Hence, a direct input (there are A-link and U.FL PCB footprints) can be utilized only instead of one of these onboard input stages. More particularly speaking, the AYA 5 A-link input can be used only instead of the onboard USB stage. In this case, the onboard USB stage should not be completely abandoned, as it can still be used indirectly, by routing its A-link output "back" to the AYA 5 A-link input, with an external cable (this takes appropriate firmware, too).
As for the Holo Red and A-link input compatibility, there is more information in this topic.
https://www.audialonline.com/community/topic/holo-audio-red-and-a-link-compatibility/