Heat heat and more heat??

Good! The 100 ohm figure is easily high enough, regardless of what the number is in the amp, to make it unlikely that the sub connection damaged the amp. That resistance would have limited the current through the roundabout path I described earlier, involving the safety ground wiring, to low levels that the amp would have had no problem with.

I don't know what to make of the fluctuating readings for the amp. Perhaps some capacitance is involved that hadn't fully discharged when you took the measurements, which might have resulted in fluctuating (and erroneous) readings. But yes, values that are very close to zero are possible, and some designs connect circuit ground and chassis ground/AC safety ground directly together (which would result in a resistance of essentially zero ohms). Although that is generally not good practice, as it can make the design susceptible to hum and other ground loop issues, especially if single-ended connections to other components are involved. In this case, though, my guess is that residual charge on some capacitance was just confusing the measurement.

Best regards,
-- Al

Thanks Al, That's great. I am actually re-thinking your idea about the XLR plug. I can indeed buy one cheap from Parts Connection and it would be easy enough to connect the ground wire to it. One question, is the xlr ground active when I am using the single ended inputs to the amp? There is a xlr switch on the amp that has to be activated in order to utilize the xlr inputs, which would not be in place since I use the RCA's.
OTOH, as you mentioned, I still believe the amp has an issue, so it will need a service. Luckily, while it is gone, I have my tube amp...an ARC D70Mk2 to fall back on. This amp is absolutely superb...I was listening to it last night; I won't be missing the ss Rowland that much while it is away, LOL.

Dave, based on the info in the manual that I referred to earlier, about XLR pin 2 being shorted to ground when the RCA inputs are selected, I suspect that the only thing that switch does is to put that specific short in place when the RCAs are selected, and remove that short from the circuit when XLRs are selected. I suspect that XLR pin 1 is always connected to either circuit ground or chassis ground, regardless of the position of that switch.

And regardless of which of those grounds pin 1 is connected to, it should serve as a suitable point for connecting the sub's ground (the black wire). Certainly if pin 1 is connected to circuit ground, and most likely if it is connected to chassis ground (although in that case there is a slight possibility of some hum resulting).

What I'm envisioning is that XLR pins 2 and 3, through which the balanced pair of signals would be supplied if the XLR connectors were being used, are routed to the + and - inputs of a differential receiver circuit. With the center pin of the RCA connector being connected directly to one of those inputs (the one corresponding to XLR pin 3), and the switch providing a ground (0 volts) to the other input of the differential receiver (the one corresponding to XLR pin 2) when the RCA input is selected.

You can verify what I'm envisioning, with respect to the grounds, by disconnecting the RCA cables and measuring the resistance between the RCA ground sleeves and pin 1 of the XLR connectors. If XLR pin 1 is connected to circuit ground, that will measure 0 ohms, or at most a tiny fraction of an ohm, regardless of the position of the switch. If it is connected to chassis ground, you may get a fluctuating reading similar to what you previously reported when you measured between the RCA ground sleeve and AC safety ground. In that event, take a measurement between the AC safety ground pin of the power plug and XLR pin 1. If that reading is 0 ohms or very close to it, regardless of the position of the switch, it would confirm that pin 1 is connected to chassis ground, and you would also be good to go.

Best regards,
-- Al

So, after taking the reading as you suggested Al, i read 0.4 ohms on the right channel xlr input and a fluctuating reading on the left channel xlr input. When I took the measurement between the AC safety ground and the xlr, the right channel was also 0.4 ohms and the left fluctuated all over. I switched the xlr/rca switch on the amp and the results seemed to be the same.
Thanks again.

Strange that the two channels gave different results. Not sure how to explain that.

Just to confirm that the correct pins were measured, pin 1 of each of the female XLR connectors would be the pin closest to the right side of the connector, when viewing the connector from the rear of the amp.

Given those results, additional measurements that might be useful to know would be the resistance between the ground sleeves of the two RCA connectors, and the resistance between pin 1 of the two XLR connectors.

In any event, I would still expect connecting the black wire to XLR pin 1 to work, although using the right channel XLR connector seems like the surer bet.

Best regards,
-- Al