If the M6000 drivetrain was new, and the chain was new and still had the factory schmoo on it, then it should have been remarkably quiet...if the chain was routed correctly. As long as the derailleur is adjusted and indexed correctly (and the chain is routed correctly), then a KMC chain shouldn't run any louder or quieter than any other brand of chain (when new). Of course, as chains wear and lubricant disappears, they get louder. Restoration of that lubricant (liquid, wax, whatever you use) should return the chain to being quiet again. Some lubricants tend to run quieter than others, and this shouldn't turn into a this lube vs. that lube discussion...the point being any new drivetrain should sound pretty darn quiet as long as everything is adjusted and installed correctly.
I agree that photos (or even a video!) of the system on its new host frame would be helpful.