REL high-level connection and McIntosh Autoformers

Pretty standard G. Checkout the link below for diagram on page 13. That is how I have my REL connected.

REL

If you buy a REL sub, that is the ONLY way it should be hooked for music... hi-level.

The Rythmik PEQ3 plate amp also allows high-level hookup (along with line-level).

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 

Not so fast!! 
The REL high level connector is balanced...IF the amp that it is attached to is also a balanced design, then connecting the cable to the negative terminal might in fact create a ground loop. Are your MA6900's a balanced design? Perhaps they have a separate ground connection that could be utilized; if not, I would suggest a discussion with the manufacturer of the amp and possibly the people at REL before hooking up....don't ask me how I know this.;0)