The six ohm resistor was a mistake, that's probably where the problem started. The regulator is specifically designed for the 8 ohm load, the six ohm load is asking too much of it. The reason that Lionel originally went to the 8 ohm resistor was the excessive failure rate of the AC regulator modules.
Let's assume the regulator voltage was designed to dissipate 5 watts in the 8 ohm resistor. That would be 6.3 volts RMS at .8 amps. If you put a 5.9 ohm resistor in, the power goes to 6.8 watts and the current 1.07, that may be too much for the regulator circuit and it's overheated and killed something.