Heat heat and more heat??

Dave, if you happen to have a multimeter there are some measurements you could make that might provide insight into whether the overheating problem has resulted from the sub connection issue.

You would disconnect the sub and the amp from each other and from everything else, including the AC outlets, and on each of them you would measure the resistance between the AC safety ground pin on the power plug and the ground sleeve of an RCA connector. If that resistance is just a few ohms or less in both cases, it would increase the likelihood that damage resulted from the connection issue. If either or both resistances are significantly higher, say tens of ohms or more, it would mean that the cause of the overheating is most likely something else.

Although it's probably all academic at this point, as it seems probable that the amp has to be sent in for repair in any case.

Best regards,
-- Al

Hi Al, I will try your suggestion with my multimeter.
What is a little odd, is that the amp still sounds pretty good. Last night I played it for about 1 1/2 hours ( without the sub connected) and it seemed fine, except for the heat issue. No hums, no out of the ordinary noises, nothing to indicate an ongoing problem. Anyhow, like you say, I think a trip to the factory for diagnosis and repair is warranted.
Many thanks again.

Well, I measured the impedance as you suggest, Al. The subwoofer measures appx. 100 ohms...while the amp is all over the place..too weird!
Both channels seem to fluctuate..what the heck?

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.