Try this circuit…..
+——————————-> Amp Left In
Amiga |
Left Out <—+—-/\/\/\/———+———-> Mono to Monitor
10k ohm |
Amiga |
Right Out <—+—-/\/\/\/———+
| 10k ohm
+——————————-> Amp Right In
It should provide enough isolation to maintain a decent stereo signal to the
amplifier.