Why Rhodium?

"Perhaps the most interesting fact revealed by this chart is how low most copper alloy materials rank in relative conductivity. One might easily assume that alloys such as the brasses and bronzes, because they are mainly copper, are nearly as conductive as copper. This is not the case. The small percentages of tin, aluminum, nickel, zinc and phosphorus that make up these alloys degrade the electrical performance of the resulting alloy to a far greater percentage than their compositional percentage in the alloy.

One should not conclude from this, however, that brass should never be used in electrical applications. There are instances where the superior tensile and machining characteristics of brass make it a better choice than copper as long as the sectional areas are increased proportionately to achieve the conductivity that a copper part would have in the application. Size for size, however, copper is exceeded only by silver among the materials commonly used for electrical applications."

www.kp44.org/ftp/ElectricalConductivityOfMaterials.php

Couldn't copy the chart but it's at the above website.

Size matters :-)

Rhodium has the best resistance to corrosion over any current material being used for termination. I own several audio components that have been in a cottage next to Lake Erie (high humidity) for 20 years. The heat is off 6 months out of the year with almost constant 80% - 100% humidity. Rhodium is the only material that has not shown signs of surface corrosion under this condition. Silver, copper and gold all eventually broke down with surface build up. Steel connections were rendered useless and couldn't even be cleaned.

A friend of mine and I compared two RCA cables in a blindfold listening test against each other. They were connected between a CD player with a direct output to the inputs of two mono block amplifiers. The cables were two meters long with both using identical wire with no insulation. One set of cables was terminated with a gold plated RCA plugs with soldered connections. The other was terminated with a RCA plug that was rhodium plated, set screwed and soldered. Both of us picked the same interconnect as our favorite over two separate listening sessions. We gave the rhodium plug a slight edge in the area of sound stage size and 3 dimensionality. This is, obviously, not a perfect test of plating comparison. It may say as much about the rhodium set being set screwed.

Willoamp, industrial gold alloys do not oxidize and are considered extremely corrosion resistant. That's why gold is used for virtually all mil-spec and industrial purposes. There are a couple of limitations: 1) gold is not terribly wear resistant, so may not be the best choice for multiple mating cycles. 2) for cost purposes, some cable manufacturers use a thin gold wash rather than a properly prepared and applied gold plating. 3) gold plating can can be problematic if exposed to extreme thermal cycles if the plating surface has not been prepared properly.

Electrically, the best way to make cables is via a crimp connection using a die designed specifically for the cable and connector in question. What you want is what's called a gas-tight connection, and you can't get that via a screw collar. Solder joints are OK, but you need to clean the flux off of the conection carefully because it is hygroscopic and attracts water. Also, solder joins are somewhat fragile and can crck or break under the strain of multiple connections (mating cycles).

I still seriously doubt that anyone can hear the difference between any type of connector plating, unless you are running extremely high current through the lines. And even then...

I still seriously doubt that anyone can hear the difference between any type of connector plating

Does this indicate that you have not tried plugs with different platings? If you have which ones? If not then you have no first hand experience, it's all just speculation. Not really helpful to the OP.