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. 

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.