With most amps having balanced or bridged outputs connecting the black wire to a chassis screw will work well, although conceivably there may be a few cases where a loud hum would result. What will certainly work well is to obtain an RCA plug, solder the black wire to its ground shell connection point (leaving the center pin unconnected), and inserting the plug into one of the two RCA input connectors on the rear panel of the amp. That would establish a connection between the black wire and the amp's circuit ground. The red and yellow wires would both be connected to a + output terminal of the amp.
Regards,
-- Al
Regards,
-- Al