I had this issue when I went from a stereo amp to pair of mono blocks.
Rel advised me quite clearly that you need two Rel subs for mono blocks, one for each amp.
So I had to purchase another Rel sub.
I initially got a S3 but have since upgraded to a S/812.
Yes, you can hook it up like this with no problem. If you do get hum, you can either hook up a single ended cable like they recommend in that link, or you can connect the black to the M1200’s ground post.