"“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.
