The latest Audial A21 amplifier, along the more usual unbalanced RCA, includes also balanced inputs, and some S5 owners already asked about the preferred connection between these two units.
Most simply told, my own preferred way is to use a balanced connection. So, S5 XLR output to A21 XLR input.
Those with transformer coupled RCA output however can also make such a connection, with appropriate cable using RCA connectors on one and XLR on the other side. Since the transformer coupled RCA ground is floating, it should be wired to XLR pin 3 (negative), and RCA hot should be wired to XLR pin 2 (positive). The S5 ground terminal beside, which is a chassis ground, can be used to connect XLR pins 1 of both channels XLR.
It is normally possible to use such an approach to connect the S5 transformer coupled RCA output to any other amplifier with balanced XLR input.
In addition to the above scheme, when using S5 and A21, instead of using XLR pin 1, the ground connection can be achieved by simply running a separate wire between the two devices’ ground terminals.
Also, since A21 balanced inputs use Neutrik “Combo” connectors, it is also possible to use a dedicated cable with RCA at S5, and TRS (three contacts) 6.35 mm phone jacks at the A21 side. In this case, the RCA ground should be wired to the ring (negative) and RCA hot should be wired to the tip (positive) of the phone connector.
In this case, since the sleeves of the A21 phone sockets are not connected to the chassis ground but to the audio ground (so they are appropriately routed to accept also unbalanced sources, by using two-contact TS phone connectors), it is definitely recommended to use a fully separate wire to connect two chassis grounds by using GND terminals at both units (and not TRS connector’s sleeve at A21 side).
Both above-described schemes, with either XLR or TRS phone connectors, will provide a real balanced signal from S5, just like it employed the XLR output connectors.