What is the reason for my speakers having an audible buzz?

Ground issue. You're gonna need to do a little trial and error tracking it down. It could be the interconnect. Or it could be the RCA connection itself, or somewhere inside the amp. It could even be things plugged into different outlets. Or maybe even something else.

Clean the RCAs real good. Both ends, male and female. Probably not this but since nobody ever cleans anything you need to do it anyway so start with this.

Thank you for responding. I'll try your advice.

Most likely it’s the insulation washers around the rca connectors. If the rca connector comes in contact with the chassis you get a buzz. When you touch the barrel of the cable you are reseating the washer and the buzz goes away. Double check by trying a cheap rca cable and see if the same thing happens. If not, it could be the MIT cable.

Ok, thanks GS5556. I will try that.  Also, another thing came to me and I can't think it through. All my components are plugged into one electrical outlet except for the power amp. It is plugged into an outlet on the other wall. Both outlets are isolated from the rest of the house electrical system, each going to their own circuit breaker.  But now what I'm wondering is : the components are all grounded via the common house elec. ground because they are all 3-prong plugs. But the Old Dynaco amp has only the 2-prong plug, making it not grounded. Has anyone had similar circumstances and know how to safely ground an old vintage amp?     James