Should I always switch off the PR/01 using the main switch at the back?Updated a year ago

This is not necessary. The PR/01 comes with power management and will only consume 0.3W while in standby.  

Standby can be activated by pressing the VOL/SOURCE button until the LEDs go off. Pressing it again will resume normal operation. If left unattended, your PR/01 will automatically switch to standby mode after 30-45 minutes and display a red standby LED. From this mode, the PR/01 can wake up automatically when playing music. 

 When leaving PR/01 unattended for longer times (like a weekend or holiday), it is still a good idea to switch off mains at the power outlet or rear switch.