I disconnected the preamp and the buzz was still there.With a 200 watt power amp I would think you would need to have shorting plugs plugged into the inputs for a true test. You need to get a pair of shorting plugs and try the test again.
If the buzz is gone or greatly reduced, shorting plugs installed, my next question would be are the ICs you are using between the preamp and amp shielded or non shielded?
(I alternate between a pair of modded Klipsch Chorus and Klipsch RF-83.)What is the efficiency rating of the speakers? The speakers and amp may not be suited for one another.
With the amp isolated from the preamp I see no way a ground loop could exist..... But you can rule that out by installing a ground cheater between the plug and the wall receptacle.
Also want to add you described the sound as a buzz, not a hum.
Is the amp new or used?
.