My experience is with IcePower based amps, specifically ones using the ASP series Ice modules. These modules have 40 VDC on their speaker output terminals in relation to amplifier ground....I was not aware of that. If that is the case with your Peachtree Grand then, yes, as the person you quoted stated you would need to have capacitors in series with the red and yellow wires. Assuming, that is, that the sub itself does not have capacitors in series with its high level inputs. (When you said that "Rel's new subs do not have the issue," I'm not sure how they could be immune to 40 VDC at their inputs unless their inputs were AC-coupled, presumably with capacitors).
Have you asked Peachtree if the Grand Integrated has a significant DC voltage such as 40V on its outputs? FWIW, its manual does not have any cautionary statements regarding the presence of DC on its outputs. Also, they would be the ones, not Sumiko or REL, who presumably could best advise as to the suitability of using an RCA shell for the ground connection.
Also, you could use a multimeter to measure whatever DC may be present. With no signal going into the amp, you would measure between each of the output terminals and the shell of an analog RCA connector.
And if you have or can obtain a multimeter, by measuring for continuity or resistance you could use it to determine if on the cable your dealer supplied the ground connection of the Neutrik connector is connected to the ground sleeve of the RCA plug, and that the center pin of the RCA plug is connected to nothing.
Good luck. Regards,
-- Al