Same problem here. Upgraded the RCA like you and then manually adjusted the sensitivity of the sub to get it to go into Standby (red light). Had to fiddle with the settings a bit before I found the right setting.
Sometimes it is still slow to go back to Standby once it has been working for a movie. Mine is plugged into a voltage regulator/line conditioner. All HDMI w/ ferrite cores here for other components.
Maybe the RCA is sensitive enough to pickup interference if there is another cable line crossing it.
Most of the time...mine goes to Standby now, since the upgrade of the RCA, the tweaking of the sensitivity settings, and the output on the back.
That is a horrible response, I know, but messing with it is all I know to do.