I don't know what "caps on the black wire" may refer to in this case. But an approach that should work fine is to obtain a simple RCA plug, perhaps at Radio Shack, solder the black wire to the ground sleeve connection point of the plug, while leaving the center pin unconnected, and inserting that plug into an unused RCA connector on the integrated amp. That would establish a circuit ground connection between the amp and the sub. Don't connect to one of the galvanically isolated digital inputs on the amp, though. And connecting to an analog output jack might be slightly preferable to connecting to an analog input jack.
Another approach that might work ok, depending on the internal grounding configuration of the amp, would be to simply connect the black wire to a chassis screw on the amp.
Good luck. Regards,
-- Al
Another approach that might work ok, depending on the internal grounding configuration of the amp, would be to simply connect the black wire to a chassis screw on the amp.
Good luck. Regards,
-- Al