HDMI boards seem to be a pretty common failure point in many AV processors and receivers. I don't know about issues with Krell in particular. You might try posting the AVSForum, there's more focus on home theater in that forum.
KRELL 707, HDMI input and output (including audio and video) does not respond!
Fault description: My KRELL 707, HDMI input and output (including audio and video) does not respond! Whenever HDMI input is used, the machine display shows waiting for audio, and the TV shows no signal input. However, the optical front and coaxial input are normal. I found that some 707 users on the forum also encountered this problem. Is it a common problem? How to solve it? Thank you