The 651W features APD (Auto Power Down), meaning that it will switch off automatically after 30 minutes if no audio has been playing.
The 651W also features APU (Auto Power Up), allowing it to automatically switch on when it senses an audio signal being played into it. However, it will only auto power up if it has previously auto powered down.
The reason for this is that if you have manually switched off your 651W, then we assume that you wish it to remain switched off.
You can also switch the 651W on/off using the Stream Magic app when used with a Stream Magic 6 - see FAQ here