Krell Service - speakers buzzing

I recently started experiencing a buzz in both my main speakers, After lots of troubleshooting, I believe the issue is my pre-amp. When the pre-amp is on or off/stand-by, the buzz is there. When actually unplugged, the buzz goes away. It is a Krell KAV-250p. I bought it used in 2013. My questions:

1. Why is it causing buzzing? I am more curious than anything.

2. Is there any point in trying to replace the cables between the preamp and amp (XLR type)? When I unhooked them the buzzing went away, presumably because there was no longer a connection between the pre-amp and amp. My assumption was since both speakers are buzzing, that is not the answer.

3. If you all think the pre-amp is the issue (and there is nothing else I should try in terms of troubleshooting), can it be serviced? Is it worth servicing? I like it. On the other hand, always fun to upgrade a piece of equipment, except everyone tells me the KAV-250p is a good unit.

Let me know if there are any other details I can provide. Thank you.

Jeffrey