Look on the right side of the engine, find the distributor. Entering the body of the distributor at about 5 o'clock are the two wires that connect the distributor pickup to the ignition amplifier. Theinsulation will look intact, but often the wire strands are broken by the normal movement of the pickup mounting plate as the vacuum advance operates.

If you have someone crank the engine and reach down and gently bend the wires, usually toward the distributor body, you might be pleasantly surprised.

 

No, the XJ6 Series III amplifiers rarely fail. Since you've tested the coil (by substitution), and you havevoltage through the coil ballast resistor,the next likely candidate is the distributor pickup. If my trick doesn't make spark, test the pickup for continuity and you're likely to find it is open circuit.