REL high-level connection and McIntosh Autoformers

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)

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.

nmmusicman 8-15-2016 1:10 pm edt
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.

I agree with the cautions expressed by Daveyf, although I believe the possibility of a problem arises because the high-level inputs of a REL sub are NOT balanced. The concern is that the black wire presumably connects to the sub’s circuit ground, which in turn connects to the AC safety ground of the sub’s power cord through some unknown and probably fairly low impedance. The negative output terminal of an amp having balanced outputs has a full amplitude signal on it, just as the positive output terminal does. Connecting the black wire from the sub to a negative output terminal of such an amp creates a path from that negative output terminal through some probably low impedance between the sub’s circuit ground and its AC safety ground, then through the AC wiring to the amp’s AC safety ground, then to the amp’s own circuit ground through whatever impedance exists in the amp between it’s AC safety ground and its circuit ground. In other words, depending on the internal grounding configuration of the amp and the sub a ground loop issue may be created, or even worse, the signal on the amp’s negative output terminal may be shorted via that path to the amp’s circuit ground, resulting in excessive current and possible damage.

Also, I would not assume that the results of connecting a REL sub to a McIntosh tube amp having output transformers (not autoformers), such as an MC275, would be applicable to a McIntosh solid state amp having output autoformers.

Best to check with McIntosh, as Davey suggested. For a McIntosh solid state amp having separate negative output terminals for each of its output taps (the OP’s MA6900 does not; its only negative output terminal is a single "Com" terminal, which is used in conjunction with the positive terminals of all three output taps) it is conceivable to me that the black wire from the sub might be best connected to whichever of the three negative output terminals (2, 4, or 8 ohms) corresponds to the amp’s circuit ground (and it might only be one of the three that is connected to the amp's circuit ground), rather than to the negative output terminal that is being used to drive the main speakers.

P.S: I realize the OP probably resolved his or her issue 6 years ago, but I’m posting in case anyone else facing a similar issue reads this thread.

Regards,
-- Al