Probably mains hum from the transformers. It has come up here before. The windings will be vibrating. It can be blamed on poor mains supply or poor transformers. If it is you’ll struggle to stop it, even with new power supplies. You’ve probably paid a pretty penny, it’s for you to decide whether you think it’s OK to have humming speakers. It may come and go at different times of the day, days of the week.

I’ve only heard of it on 8ks, never 6ks or 7200s.

They are big transformers but there’s no need for them to hum...