Dual subwoofer setup to an Esoteric Integrated Amp

I have an Esoteric F05 running a single REL subwoofer using a high level config. It is setup with yellow cable going to the left positive speaker terminal and the red cable going the right positive speaker cable and the black cable grounded to the F05 ground post.  I’m not sure if this is the way it should have been done, but it seems to work.  
 

I would like to add a second REL but I am a little concerned, not completely understanding the electronic design of the F05. I know the F05 is a balanced dual mono design, but I’m not sure if it is a differential design.  This causes me to worry about what to do with the black cable on each of the two REL high level cables I need to use. 
 

if anyone has any experience connecting subwoofers to an Esoteric integrated amp and is willing to help, I’d greatly appreciate it.  I wish I knew a bit more about the electrical nuances of amp design. I just want to make sure I don’t damage my equipment. 
 

Thanks,

Dave