Hi, is the noise the same no matter where you plug in to mains in your house? For example, does the upstairs mains ring sound any different to downstairs? It seems you've already identified that some equipment is dumping noise onto your mains ground signal or similar so if audio equipment is using that for a reference it seems it could become audible. An electrician is likely to compare the earth connection once mains to a copper stake in the ground outside (or at least they did at a studio install I was working on) for a differential but not likely to put an analyser on the mains to look for noise. There might be someone more mains savvy one here who can advise.
A DI box can lift the ground connection just like the Kemper does, but this is for prevention of ground loops where errant 50/60Hz signal flows partially through mains socket to socket, then via audio cabling between the bits of equipment powered for those sockets (I'm perhaps telling you what you already know here). If the noise is part of the audio signal though, ground lift won't touch it as you've found. A passive DI box is transformer-based and will provide physical de-coupling in addition to ground lift, but again if the noise is already part of the audio signal, it will get transformed too. Still worth a try if someone can lend you one.
It might be possible that a mains RF filtered socket extension would help. Quite a few affordable mains extension leads feature this, filtering errant high frequencies that can sometimes cause issues (I have some D-Link powerline network equipment that operates by dumping 2.5MHz signals over the mains in my house, which is great for networking but not so much for some of my audio equipment).
Best of luck with it.