Very interesting, thanks! Rick sent me a PM with the same tip, which is why I tested complete removal of SpeakerLink, as well as a different circuit. No change, although based on the description in the other thread, it's still possible this noise is riding the power lines somehow, in which case I have not yet ruled out the issue being similar.

I'll have to find access to a completely different circuit... maybe I'll run a long extension cord from my pool gear, which is a separate panel from the main house!

Main System: MiniDSP SHD (Roon, Dirac, Volume) > G61R > DSP5200 Speakerlink L/R, 2x SVS SB3000 Subwoofers
Office1 System: Genelec 8030c L/R, Genelec 7060B Subwoofer
Bedroom System: DIY Sure 2x100w Class D Amp, ELAC DBR62 L/R