If they are plugged into separate outlets try plugging them both into the same outlet. You may be getting a ground potential hum even if both outlets are on the same circuit.
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If you are using the balanced/mono setup, be sure to check your setup connections. Make sure you are using the correct input and output connections on the amp and be sure the toggle switch on the back is in the correct position. It's been a while since I owned a pair, so I went to the Monarchy website for clarification. You should be using the balance input, switch in the down position and your speakers should be connected to the two RED speaker binding post, not to one red and one black as it would be for stereo. The top left red speaker binding post is for your positive lead and the top right red binding post is for your negative lead.
Check the manual or contact Monarchy to verify the above.
Agreed with Rello that plugging both amps into a single duplex or quad outlet will tell you if you have a ground loop problem. Lastly, try lifting the A/C ground using a cheater plug (3 prong input, 2 prong output).
I had a pair of Monarchy SM-70s (not the pro model) and never could get rid of all of the hum. I really liked the amps, but the hum bothered me in my near-field setup.
Just wanted to thank you all. I solved the problem by lifting the ground on both amps. But a strange thing happened in the process. I plugged both amps into my PS audio Duet and didn't have hum but I got a muffled output from both amps even when plugged into separate banks. I unplugged one and plugged it into the wall socket and things were all good. I have a Furman pf 15 elite, so I decided to try both amps plugged into the high current outlet and this proved to be a great move. Absolute quiet. What gives with the Duet?
I had the same problem with my SM-70 Pros, but hated the idea of hanging a cheap crappy cheater plug off the end of my nice power cables. It's a pretty easy mod to take the top off the amps and disconnect the ground wire internally, so that's what I did. Alternatively, there are power cords with removable ground terminals, or you could even snip them off if your cords aren't expensive. Cheater plugs are great for diagnosing ground issues, but I would never leave them in the circuit for serious listening. Why add another potential choke point?