REL subs with Rowland Amplifiers

Richard, I think if you always have your RCA ic connected to your preamp, then you are always listening to the low level connection...by default. I am under the impression that the connection to the low level input is done by inserting the RCA ic and connecting it to the preamp. Regardless of whether you connect to the high level as well. IOW, you have perhaps ONLY listened to the low level connection for all this time! 
Now that you have done away with the RCA ic, you are for the first time listening to the high level. ( Again you are not establishing a good ground, resulting in the hum!) Here's a suggestion.. hook up just one sub as I do with my Rowland. The red to right positive, the yellow to left positive and the black to either negative connector. ( Make sure the RCA ic is NOT connected). This should be tried with the original cable supplied from REL...NOT the Signal cable ( as presumably this was wired for a mono hook up). 

IOW, you have perhaps ONLY listened to the low level connection for all this time!

Note the reference to "volume all the way down" in Richard’s statement that ...

I had a RCA cable going from both RELs LFE (volume turned all the way down) to the preamp out, while connecting the subs to the amplifier...

Also, regarding ...

Here’s a suggestion.. hook up just one sub as I do with my Rowland. The red to right positive, the yellow to left positive and the black to either negative connector.

While this **might** work, depending on the internal grounding configurations of the sub and the amp, given that the amp has balanced outputs I would strongly recommend against connecting the black wire as you have suggested. The black wire is most likely connected within the sub to the sub’s circuit ground, which in turn is probably connected through a resistor having a relatively low value (i.e., a small number of ohms) to the sub’s AC safety ground. Which in turn is connected to the amp’s AC safety ground via the AC wiring, and the amp’s AC safety ground is in turn connected to the amp’s circuit ground through some unknown but probably low impedance within the amp. Since the amp’s outputs are balanced, a full amplitude signal would be applied to the black wire. Depending on the resistor value and other variables involving the internal grounding configurations of the amp and the sub any number of bad things could happen, including hum, the resistor being burned out, the amp going into protective shutdown mode, or the amp even being damaged. As I said, it **might** work, but IMO it’s not a good approach even if it does.

Regards,
-- Al

Richard, I would defer to Al’s points..and therefore not implement my suggestion. OTOH, Al do you have any idea as to why my balanced JR model 8T has no issues when connected in the manner that i suggested? ( which also was the way that REL had suggested to me to do the hook up when I first acquired my sub).

I did notice that Richard turned down the low level connection...BUT I believe that the connection to the low level bypasses the connection to the high level once the RCA ic is connected to the preamp/sub. Since he had sound, I would suspect that the sub may have been out of the system blend and the mains were being solely listened to. Any other possibilities?

davefy,
If you read my post dated 9-21-2019, "I have been corresponding with Justin Magana who is REL’s Chief Engineer. Based on his advice I twisted the red and yellow wires together and connected my REL via the high-level input. The black wire is floated. It worked! I have plenty of output from the sub and absolutely zero hum." PLEASE NOTE NO RCA CABLE WAS CONNECTED AND IT DIDN'T SOUND GOOD TO MY EARS. I clearly have listened to the high-level connection with the supply REL speakon cables. 

"The red to right positive, the yellow to left positive and the black to either negative connector." PER MY CONVERSATION WITH ROWLAND AND REL CONNECTING ANY LEAD TO THE NEGATIVE CONNECTOR  WILL DAMAGE MY AMP, SUB, OR BOTH.

I have two options:
1) use the low-level input connected via RCA cables
2) send my amplifier to Rowland and have him add an external ground screw that will enable me to safely connect to the high-level input

Richard,
Clearly the best ( and maybe the only way) to hook-up the sub to your particular amp is to have Jeff install a ground connector ( like he did for me). This is a sure fire way to success. 
Since you like the low level connection, maybe sending to Jeff isn't going to be worth it....with all of the hassle that will involve.