RCA Caps

Kalali, a shorting plug will have electrical effects, by lowering the input impedance of the unused input to essentially zero ohms. Low impedance circuit points are inherently less susceptible to pickup of electrical noise than high impedance circuit points, everything else being equal. Including pickup of noise that may even originate elsewhere within the same component, that may in turn find its way from the unused input path to the signal path that is being used, or to a circuit point (such as in the power supply) which can affect that signal path.

As was mentioned earlier, and despite some disagreement that was expressed, caps that do not short the input will do nothing other than perhaps reducing oxidation or corrosion of the ground shell that may occur over time, and keeping out a few dust particles.

Regards,
-- Al

https://www.ebay.com/p/8pcs-RCA-Cap-Protector-Dust-Proof-Brass-Gold-Plated-Shielding-Caps/1261404073...
That's what I using on my HT receiver unused inputs and remove the center pin for the unused outputs 

Like the Cardas Caps they don’t having to be shorting type caps to be effective.

These are good for nothing sound or noise wise.

Like Al said,
"a shorting plug will have electrical effects, by lowering the input impedance of the unused input to essentially zero ohms. Low impedance circuit points are inherently less susceptible to pickup of electrical noise than high impedance circuit points" " caps that do not short the input will do nothing other than perhaps reducing oxidation or corrosion of the ground shell that may occur over time, and keeping out a few dust particles."

Cheers George

Another solution is to chew a piece of gum along with shreds of aluminum foil and copper shavings. Once chewed, remove the metal reinforced gum and stick it on the unused RCA plugs. Done!

Georgie Boy, if I were you I'd seriously consider modifying your post to say, Cardas caps do nothing for the sound that I'm aware of. There, that's more like it!