Earth ground vs. chassis ground. Uck.
Back in the 70s Yamaha made a revolutionary pro live sound mixer called the PM-1000. Very clever audio design and totally modular so any of the 16, 24, or 32 input channel cards could be serviced separately. Cool. But, earth ground and chassis ground were separate, and ground from the mics brought in on Pin 1 through the modules then to earth. A floating ground. Not only was this hum-prone, if a grounded mic case ever contacted a hot AC source (it happens) it would blow every ground trace off every module on its way to earth ground. After having to same-day air freight ($$$$) an entire 32-ch mixer in a 3' X 6' road case yhalf way across the country to save a sold out 20,000 seat show, lesson learned. I was one a a small handful of techs in the country who was allowed to go in and modify the PM-1000s by building a proper ground bus on the inputs before anything could get to the modules. Looks like Krell may have made a similar design choice/mistake. That grounding scheme only works when everything else also has a floating ground. And that's a big assumption. If an upstream connection earth-grounds Pin 1, you have an instant ground loop, and a differential impedance which means voltage, which means hum.