Toroids are prone to mechanical buzzing, vibrating, when fed from AC mains with DC on the Line. Especially large VA Toroids used in power amps. For smaller VA toroids you can buy a plug in DC blocker that will filter out the DC on the mains and in most cases stops the loud mechanical buzzing, vibrating, sound.
Designers like Pass and Bryston install DC blocker filters in their equipment that use Toroid power transformers. Many do not... I see some ARC equipment is now using Toroid power transformers. I imagine ARC is installing DC blocking filters in the equipment.
Unless Krell has changed their design recently they do not install DC blocking filters in their equipment. There are a few threads on Agon about big power Krell Amps that have Toroid power transformers that mechanically buzz, vibrate, quite loudly. The owner tries a plug in DC blocker but it doesn’t stop the loud buzzing.
I believe the problem is the plug in DC blocker is not designed to handle the higher draw of the big VA toroid transformer(s) in the amps.
EI type powers transformers as a rule are not affected by DC on the Line.
DC on the Line: