It can’t be that difficult. Pharmaceuticals are tested that way. Either something works comparing to placebo or not. And the variables are really complicated there. We bring people to the same room with the same high quality sound system, set of recordings and procedures agreed by a venerable audio society. We have our control cable, placebo, something off the shelf. We compare to it other cables with the same L, R, C. We ask testers to judge the sound reproduction accordingly. Neither experimenters nor testers know whether tested cables or control are used. If the results are near 50/50 then we know that if L, R, C are the same, no other variables matter. Pure physics. Bob Carver did it with amps, surely somebody must have checked it with cables?
Then of course another study could be done how cable L, R, C influence people’s judgements. And how expensive, or not, is tuning those three parameters.