It sounds like you're dealing with a frustrating issue! The hum you're experiencing could indeed be related to grounding, but there are a few other things you might want to check as well.
First, double-check the entire signal chain. Even though your connections are balanced and cables are shielded, try swapping out the cables with others to rule out any defective ones. Additionally, ensure that all connections are tight and secure.
Another potential culprit could be electrical interference from other devices. Make sure your monitors aren't too close to any power supplies, routers, or other electronic devices that could emit interference. Also, try plugging your equipment into different outlets or a power conditioner to see if it mitigates the hum.
If these steps don’t resolve the issue, it could potentially be an internal problem within the monitor itself. In this case, reaching out to KRK's customer support for troubleshooting advice or possible repair options would be advisable.
Lastly, consider checking if your studio setup is on a properly grounded electrical circuit. Sometimes issues in building wiring can cause these kinds of problems and might require an electrician's expertise.
I hope one of these suggestions helps solve the humming issue with your monitors!