One ear in when punters are on prem - lets me catch calls, listen to machines, and hear when fools are wandering in.
Two ears when I have the house, or production doors are shut and energy needed.
No ears when I'm concerned that stuff might be interesting.
No ears and amp + speakers when punters are on prem, door is closed, energy needed and a sonic DND is required to discourage interuptions. And A+S+volume, when fast energy music is needed to kill the last jobs in the evening (helps me to not hear how out of tune I can be).