REL subs with Rowland Amplifiers

Richard, If Jeff told you to use the low level input, perhaps he was suggesting that in order for you not to have a problem hooking up to his amp. That would be my impression. You must have told him of your issue with the hook-up from your OP. 

Also, while the two inputs use the same board, that doesn't mean to say that the signal that board receives will be the same...in regards to low input vs high input. 

daveyf,
Jeff Rowland and I talked for several minutes about REL, but it isn't my place to post our discussion. 
I reached out to REL again this morning to make sure I wasn't missing something regarding how the subs were connected. I have no problem staying with the low-level input, because that's what sound best in my system. 

Update from my last couple of posts. I haven't sold my Signal cable, so I decided to try the high-level input again. The last time I did, 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 via the Signal speakon cable. REL told me to do that as a way to ground the subs. Today I decided to try the high-level input again. I still used the Signal cable, but this time I didn't connect the RCA cable from each subs LFE input to the preamp as a means to ground the subs. This time I received an audible hum. The weird thing is with the standard REL speakon cable I don't hear a hum, even though it's connected the same way as with the Signal cable. Based on my findings, because of the "grounding" issue, in my situation I don't think using the high-level input is optimal. In my situation using the low-level input is clearly better. I will contact Rowland tomorrow to see if it's worth sending my amplifier back to them to add a grounding "screw". I think that's the only way to have a fair comparison between the two connection methods. More to come!

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).