Does that still work with modern amps ? My recollection was that the -ve terminal (black) on most amps was "ground" these days, so the -ve of right would be the same as -ve of left and so you would only get the left channel.

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8