I haven't located the pilot light in the schematics yet, but, while not digging into the chassis, and going from what's available on the outside of things, (and assuming the power supplies are all intact), none of the signal level outputs are AC coupled, so, you should be able to find mV DC level potentials on Line Out jacks, Headphone Jacks, Stereo Link Output jack, Subwoofer Out jack. You'd see nominal DC levels near 0mV +/-50mV or less. That would be a clue as to whether the bipolar supplies are up and running. Likewise, connecting to the woofer terminals, if the supplies are working (higher DC voltages), you 'd see nominal DC level on the woofer, 0mV +/- 50mV.
We don't know your skill level, but with nothing working on the KC-550, we start with the power supply, and verify all the proper voltages that run the circuits are present first. I see fuses in the output driver xstr emitter circuits on the power amp, so if either/both are open, you wouldn't get output from the speaker. The Tweeter has a simple RC circuit off the woofer output, so there wouldn't be DC level on it.
If the preamp/mixer circuits are working, you'd be able to get output from the Stereo Link Output jack from any of the inputs. The Stereo Link Input is a path to the power amp input, allowing you to bypass the mixer/preamp circuits and drive the power amp directly.
From there, if not getting any results, you're digging further into where the fault(s) are, and we can assist you further when you report back.
NOTE: Found the LED (LED1)...it's on pg 1 of 4 of the KC-550 schematics...in series with a 2.2k resistor from the +15V supply. NO LED lit could mean you don't have the positive side of the bipolar 15V supplies. If that's the case, you'd be seeing substantial DC offset on the outputs of headphone, line outs, subwoofer out, Stereo Link Out jacks.