REL subwoofer using only red cable not yellow per REL technical dept

Ground hum will occur when connecting a single subwoofer to two monoblocks, and this hum can be extremely difficult if not impossible to eliminate. If you are only able to support a single subwoofer in your system at this time you can connect it to only one of the monoblocks, though only that channel which the subwoofer is connected to will have low-frequency extension, which can throw off the stereo imaging of your system. When using a single REL subwoofer with monoblocks it is recommended to use the Low Level input on the sub, connected to preamp outputs, which will allow the single subwoofer to receive both left and right channel information.

@jacobatrel 'Jacob at Rel', thanks for joining the discussion.

Is it 100% certain, regardless of what monoblocks are being used that the low level hum will exist, or is this a strong recommendation just in case.

I'm really not concerned about having mono vs stereo imagining. I've had mono and dual subs in my system in the past. I agree two is better than one. I don't use a preamp, so the low level input isn't possible.

It isn't a 100% certainty that hum will occur when connecting a single subwoofer high level to two monoblocks, though in our experience it is pretty common, and when it does occur it can be near-impossible to eliminate. 

Thank you Jacob.

Going back to OP.  I have two REL T/9i in my setup.  I had them wired with yellow left and red right for both.  Then, I read about RELs recommendation to run them as stereo, and wired them that way.  

They suggest that you can put red and yellow together, such that you are running two channels in each sub, as they are designed.   They also suggest that if the gain is too high, and you have to turn the sub's volume low, then you can lift the yellow.  In my case, I had to run the RELs on only one or two clicks above zero, or they'd be too loud.  

Now, I have the red wire on each sub connected to one channel (i.e. one on left and one on right), and the yellow wire is not connected.   In this configuration, the volume is between 10 and 12 o'clock, and I ended up setting crossover higher than I expected.  This ended up bringing out the midrange much more clearly, and the overall sound stage opened up.  My experience matched yours.  I did also cover the bare wire to prevent any sort of short or other issue.