Is the McCormack amp one of those having balanced outputs, such as the DNA-500? If so, is the negative input to the sub (the black wire on the original cable) being connected to a negative output terminal of the amp? If so, that is probably the cause of the problem.
On a balanced amp the negative output terminal has a full amplitude signal on it, just as the positive terminal does, and that signal is in effect being connected to the amp's ground through a very low resistance when it is connected to the sub's ground via its negative input terminal. Probably the higher resistance of the original cable (it being much longer and of much narrower gauge) was high enough that the amp could deal with it.
If all of that seems applicable, try connecting the negative wire of the sub cable to a chassis screw on the amp. That is likely to fix the problem, if my diagnosis is applicable. Or alternatively, as an experiment, temporarily defeat the AC safety ground connection of the sub's power plug with a cheater plug (a 3-prong to 2-prong adapter), since if my diagnosis is applicable the AC safety ground wiring would be part of the path through which the amp's negative output terminal is being shorted to the amp's internal circuit ground (which is what causes the problem).
Regards,
-- Al
On a balanced amp the negative output terminal has a full amplitude signal on it, just as the positive terminal does, and that signal is in effect being connected to the amp's ground through a very low resistance when it is connected to the sub's ground via its negative input terminal. Probably the higher resistance of the original cable (it being much longer and of much narrower gauge) was high enough that the amp could deal with it.
If all of that seems applicable, try connecting the negative wire of the sub cable to a chassis screw on the amp. That is likely to fix the problem, if my diagnosis is applicable. Or alternatively, as an experiment, temporarily defeat the AC safety ground connection of the sub's power plug with a cheater plug (a 3-prong to 2-prong adapter), since if my diagnosis is applicable the AC safety ground wiring would be part of the path through which the amp's negative output terminal is being shorted to the amp's internal circuit ground (which is what causes the problem).
Regards,
-- Al