Krell Service - speakers buzzing

It could be an incoming power issue but just as easily could be a power supply issue and should be serviced soon. Disconnect all the inputs to see if the problem stops.  If it still happens, take it to another room or building to see if the buzz magically goes away.

Another possible thing to try is to turn off everything (and I mean everything) else in your house. Turn off any other devices on the circuit, and turn off all other breakers. If your buzz goes away you may have DC or a noisy power supply somewhere in the home.

Otherwise, no, your preamp sucks, it’s trash, and you should send it to me to save you the cost of recycling.... 🤣

Seriously though, the last thing you probably need to worry about is your cables. 

@erik_squires Thank you. This is all good advice and tests I can try. Greatly appreciated.

A simple alternative is to borrow a different preamp (or use the preamp output of an integrated amp) and see if the hum is still there. 

If the hum disappears with the other preamp, this indicates the Krell needs servicing. If the hum is still present with the other unit then you need to look elsewhere in your house for the cause, as Erik described.

I noted this particular Krell unit was released around 1997 so your unit could be up to 27 years old. Its not uncommon for the power supply capacitors to need replacement at this point, but there are also other things that might need fixing.

Throw a cheater plug on your preamp and see if it goes away. Not saying that should be a long term fix. 

"Throw a cheater plug on your preamp..."

That's a good experiment to try when you have a hum problem the first time you use a new piece of equipment or install it to a new location or setup, but my understanding of the OP's issue is that the preamp had been working just fine for the past ten years and only recently started buzzing.  That reduces the likelyhood of a ground loop issue and increases the odds that its an equipment aging problem.

I agree with @mlsstl.   Borrow another preamp and see if the problem goes away.  I was a fan of old Krell myself.  They need recapping every 7 to 10 years or sooner depending upon use. Regardless of your problem, if  you never recapped it, have it done.  Krell in Orange, CT used to service my 97 vintage models.  Better is the site below.  They will use better parts do a meticulous job IMHO.   Insane waiting times to schedule repairs with both.  
https://www.theservicedepartmentct.com.  

Thank you everyone. I am trying to figure out how to borrow another preamp. I don’t know a lot of other hifi people locally. I appreciate the link to the service center above. It’s looking that way as I really don’t want to buy a new unit if I don’t have to, although changing pieces can be fun. It’s amazing how many people like Krell and this preamp which leads me to hope it can be repaired.

Update: I tried a few tests with the cheater plug, unplugging one or the other XLR cables; no luck. Today I tried a new preamp using all the same cables and no buzz. I think I’ve done everything to indicate the Krell is the issue?