REL high-level connection and McIntosh Autoformers

There is no cause for concern. I have a REL Stadium lll connected to a MC275 Mk V utilizing the high level connection with no issues whatsoever. 

Connect the Red wire from the REL high level cable to the Right channel positive, the Yellow wire to the Left channel positive and the Black wire to either channel negative on the amplifier. Connect the Speakon connector to the high level input on the REL.

The stock REL high level cable will be wired correctly to the Speakon connector as would a reputable after market cable manufacturer that offers REL cable upgrades. I would only be concerned if the high level cable was a DIY but if you're buying the REL new, it will come with a high level cable.

Cheers 

I know for a fact that the AC input GND connection on my Stadium lll and Strata lll are not connected to anything, only the Hot and Neutral AC input is connected to the input transformer. The GND pin is not electrically connected , thus the REL ST series lll subs get their GND from the amp or preamp that it is connected to.

With the newer REL units that utilize the class D amps, I don’t know how the GND is implemented so it would be best to contact REL regarding connection to an amp that uses unbalanced autoformer output topology.

I’m fairly certain that any MAC amp that uses autoformers is not balanced on the output. Like I said, my amp uses the autoformers and there has never been an issue using several REL units.

Of course, it never hurts to contact either REL or the amp manufacturer to be 100% certain.