Unless you are using the top tier McIntosh like the 2 boxes preamp, there is no benefit to balanced. Even though almost all Mac equipment has both balanced and unbalanced connectors, they are not truly balanced. If you look inside the unit, the balanced connectors are connected to the RCA connections as well. Like I said, only their very best amps and preamps are truly balanced.
I am willing to bet you had a problem with that particular RCA cable and if you replaced it with another RCA, the hum would have been gone as well. I got this information when I toured the McIntosh factory in 2019, right from the engineers mouth. They are there for convenience only.