The REL support pages specifically recommend to use the low level inputs with monoblocks and a single REL sub:
Monoblocks (1 REL) Connection Method
"It is not recommended to use the High Level cable when connecting a single REL. To achieve stereo input to your REL you will need to use the Low Level input. Please see our Low Level input guide."
If 2 subs are used with monoblocks REL revert to the high level speakon connection method.
I use the low level connections in for my setup and it integrates fine. Some users have apparently compared the Speakon to low level connections and prefer the later. Like most things it's probably system dependent.
https://relsupport.zendesk.com/hc/en-us/articles/115004649108-Monoblocks-1-REL-Connection-Method
Monoblocks (1 REL) Connection Method
"It is not recommended to use the High Level cable when connecting a single REL. To achieve stereo input to your REL you will need to use the Low Level input. Please see our Low Level input guide."
If 2 subs are used with monoblocks REL revert to the high level speakon connection method.
I use the low level connections in for my setup and it integrates fine. Some users have apparently compared the Speakon to low level connections and prefer the later. Like most things it's probably system dependent.
https://relsupport.zendesk.com/hc/en-us/articles/115004649108-Monoblocks-1-REL-Connection-Method