Amplifier Damage

All good @rman9 

I inadvertently hooked up a second preamp output to another devices RCA output.
Our courser the speaker went pretty quiet as one source was driving 0v and the other signal, so you get a voltage divider.

But they are made to do that, or the poutoput impedance would be s closer to 0-ohms, that t50, 500, or #k ohms.

(Probably also why people with high output impedance have more cable effect than ones using a lower output impedance preamp.)

I would not fret, but it is better to ask Luxman as they know their stuff more intimately.

I’ll look forwards to seeing what they tell you.

fine, but I had the nagging question. Does the input audio signal get shunted to ground through a diode in the input amp stages when its powered off or something to protect the circuitry? 

Any inputs will be great

With a DMM one could measure the input impedance with the amp on and off.
I have never heard of an amp shunting the input to ground.
Even if it did the output impedance of the DAC or preamp would mean that the DAC would not suffer,

Subjectively one would hear the level drop if the amp that was powered off was shunted to ground. And we never hear that.