Why is my Krell fading out?

Hi sounds like you have bad output transistor(or supply Vcc, Vbe caps) that needs to be compared/tested vs. good ones when hot and when you spot problem right on circuit for various voltage drops Vbe Vcc.
See the difference?
You will have to test transistor AND all applied DC caps that supply Vcc and Vbe.
Krell runs transistors hot and hard so they don't last long there.
If you post detailed pictures of open boxes, I'll probably be able to direct you how to test the amp.

At this point, is even safe to turn this thing on?

At this point turn it on to troubleshoot only. In most cases other transistors and circuit elements are safe.
Playing music maynot be safe though.
If DC cap is blown, than you'll probably see it visually, but transisors in the circuit are checked by comparison of voltage between terminals. Terminals of transistors are ID'd from part-no or may have visible markings of E, B, C(Emitter, Base, Collector). The task is to measure on all of them voltages between terminals. Please note that the negative half-wave transistors will have negative applied voltage.

Marakanetz: Thank you for your input. A friend with considerably more experience than I have is coming over with his tester.

I will share your words with him and see where it leads.

Again, thank you!