Always on. Because I don't usually listen at Reference Level.
I read an article in this month's UK mag Home Cinema Choice where they interview the head designers from Denon and Marantz. Interestingly, the Denon design chief says that they aim for a transparent, neutral-to-the-source sound while the Marantz guy says they aim for 'musicality' - that is a deliberate distortion that 'favours' music reproduction. To some, the Marantz sound will be more pleasing I guess, in the way that tube amps sound pleasing to some. Personally, any unit that deliberately deviates from transparency to the source is as much use as a snooze button on a smoke alarm AFAIAC.
Do you have to manufacture pianos to know what one sounds like?? ;)