ARC Ref 6 - Is it normal?

I’ve never seen that issue. A conditioner doesn’t do anything if there’s an issue on the other side of the outlet. 

Low voltage isnt isn’t as bad as too much (a surge). I know that if you run an AC on half power for prolonged periods*, it can damage the unit, but I don’t know about preamps. 

* the 240V wiring to the outdoor compressor was wrong and I had to replace the unit. 

Yes, I find this on Ref 6 user manual page 5. 

Quote:
When sensing low line voltage, the preamp displays “Low Line” and automatically goes into “Mute”. This condition will persist until the line voltage returns to a safe operating level,. The unit will remain in “Mute” even after “Low Line” no longer appear. 

A conditioner doesn’t do anything if there’s an issue on the other side of the outlet.

That depends on the power conditioner. Some can regulate AC voltage and thus keep it at a proper level even when the line level is low.

I aware this low current problem came from a heater was using when I switch on my Ref 6. And I just check this heater, the output is 1,000W !!
When I don’t use it, there warning message does not exist from my Ref 6. Everything is clear now. THX!