About the previous REL S3 and S5 series and lower output via RCA ... compared to T series.
My hifi dealer a few years back told me the exact same problem and even showed it to Rel rep in his showroom by switching the same sub cable between a T9 and S5. Even then they would not admit there was a problem. The reason is the input sensitivity on the old S series is just too low. My dealer persisted and persisted to no avail but Rel took back the Rel S series he had in stock and replaced them with T series since he was fed up.
Even knowing this I purchased dual S5/SHO 3 years ago. The problem was there but I managed to work around the problem by adding 8db to my sub outputs (on my processor). That resolved the problem in my case. I can only hope Rel solved this design problem in their newest S serie and I assume they did because the new S amp has more output now.
I really liked the Rel S subs once dialled in correctly but a year ago sold them to free up some living room space.
Stereo:Lyngdorf Tdai 3400,Q Acoustics Concept 500,Innuos Zenith MK2 (Roon),Purepower+ 1500,Vicoustic panels
HT: Lyngdorf MP60,3 x Lyngdorf Sda 2175,Arendal 1723 Monitor S/Center S/Surround S,2 x B&W DB3D,Sony ubp-x800m2,OSMC Vero 4K,Propower 1050,JVC ...,Stewart Studiotek G3