mark levinson 380s

How do I turn off the power completely

pull the plug.

what is wrong with having the unit in standby ?

i'm not sure, I've heard leaving on standby for a long time could be bad for the component.

"“off ” mode—it is designed to standby be connected to the AC mains at all times for the best performance."

- found this online for the 380s

Standby mode on audio components I own/previously owned, just means the circuit is off, but ready to be powered on so it can reach optimal operating temperatures. You should be able to safely unplug the power cord from the wall/power conditioner if need be - consider how much power is used during standby in the manual.

There is truth in the idea that allowing Class A amplifiers to "warm up" improves sound quality. After about 5-10 minutes for small-medium components and 15 minutes for large power amplifiers. Once the operating temps are at the engineering standard, there is no benefit (in my experience) to keeping your gear powered on indefinitely. 

I run Hegel amps and found it interesting that both my H390 and H590 had a standby mode and it would engage after the amp sat unsued for about 30 minutes.  My H30 amps have no standby mode available.  If I don't manually turn them off they say fully powered.  I trust the engineers had a good reason for this.  The manuals do recommend turning the H30 off when not in use.   Above my pay grade.