The description that you have referenced is to be used if the source amplifier is a class D amp, or likely any amp that does not have the low side of the amp output referenced to ground. An amp with one side inverted could also present a problem if you were using the .1/LFE input as well as the high level outputs.
The REL Subs seem to have the ability to use both the high level inputs via the neutrik connector as well as the .1/LFE channel simultaneously.
If used in this manner, the low sides likely need the same reference ... just a guess without seeing the schematic of the REL.
The schematic shown indicates the low sides of the amp are ground referenced. The amp is NOT a class D. Put an ohm meter on one of the input RCA shields and one of the amp low side outputs. As long as these are connected, you can hook this up normally without using the .1/LFE connection if you don't want to. If you plan on using that connection directly to the OPPO, I don't see a need to leave the low side of the high level inputs floating. If you left them floating, you would have problems if you disconnected the .1/LFE channel.