I have a buzz

You probably have a ground loop. Assuming that is correct you need to identify the source. I suggest you not only disconnect but unplug everything connected to your amp (except the speakers of course). Turn the amp on. Any buzz? I suspect not, but if you do its likely something is wrong internally.

Assuming there is no buzz, plug in a component (do not connect it with IC's). No buzz? If not the turn off amp and connect IC's. No buzz. Repeat process with other components until the buzz appears.

When the buzz appears attach a cheater plug to the PC on that component replug in in and see if the buzz goes away. If it does you are in hog heaven. It is not unsafe to operate your system so long as the amp is fully grounded.

If nothing eliminates the hum and the hum is really low in volume, i.e. heard only with your ear at the speaker grill, you might have just heard the amps noise threshold. This is not uncommon with very efficient speakers and its not a big deal.

Your power line may have a ground fault.
Try to block loose ground wire with cheater plug or ask electrician to take care of the problem.

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?
.

What is a cheater plug? And the Klispch are very efficient.

This is a cheater plug:
http://en.wikipedia.org/wiki/Cheater_plug