it's a bitch when everything is pcb mounted.
i, fortunately, know how to use an exacto knife and a soldering iron.
baring that, you'd have to unsolder the rca jacks and replace with free floating ones. then you'd add the resistor and a ground wire. heaven help you if you lift an etch.
if you know how to make cables, all you would do is add a resistor between the centre pin and the centre wire. but then you can't use those cables anywhere else. you'd have to label them for future reference.
let's do the math:
a 3 dB drop would have to be 1/2 power, and a 6 dB drop would be about 3/4 power... a 50k ohm pot woulkd therefore have to be 3/4 of 50,000 = 15,000, so i would add a 15k resistor in the signal path.
in the case of adding the resistor to the middle tang, you could never get it completely quiet, (same as if you put the resistor from one tang to ground [break the ground going to the resistor and inserting a resistor in it's place])
the easiest way would be to put a pot in it's place (of the resistor), adjust for best sound movement, remove the pot, measure the resistance, replace with same value resistor.
do they make 1% metal oxide 1 watt and 2 watt resistors?