ARC Ref 6 - Is it normal?

It sounds like you might have low AC voltage. Have you checked it?

cleeds

No, I haven't check it and I was not award about this at that time.  I connect all my equipments to an Audience sR6-TSS power conditioner. When I play music last night, it seem everything OK. 

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.