Another buzz issue

Hi All, I recently updated my preamp to a Marantz AV7706. The Marantz has a 2 prong plug and after extensive trial and error have determined the hum must be a grounding issue. I have separate amps on all channels (a total of 4 amps with 3-prong plugs and a Marantz, 6 channel amp powering 3 pair of overhead fronts, sides and overhead rears for (ATMOS) configuration that, like the preamp, has a 2 prong plug. I don’t want to put cheater plugs on ALL of my 3 prong plugs so was wondering if/how it’s possible to “ground” - or convert - the 2 prong plugs to 3 prong? FWIW - I have a dedicated circuit for the HT/sound system, almost everything is plugged into a Furman AC “conditioner” and all plugged into the same circuit. There are NO coax runs. I’m sure there is another thread on this topic somewhere but am looking for guidance. Also, all the interconnects are in shielded, RCA (my 4 old Hafler 9300’s don’t take XLR). To rule out the cables I put in some old AQ shielded cables with no luck. I will also mention that there are a few auxiliary components (subwoofer, Amazon cube and and Apple TV that are plugged into a Panamax which is plugged into the same outlet/circuit - I just don’t have any more outlets available on the Furman. I have tried to troubleshoot this for over a year and have had 2 installers come out with no luck. At first I thought there was a problem with the Marantz but swapped it out for a new one (same model) so doubt that’s the issue.