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

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.  

adding a link. https://relsupport.zendesk.com/hc/en-us/articles/115004649048-Stereo-Subs-Connection-Method

Here it says you can use either red or yellow.