Hi John:
There should be no issue with a hum from your setup. And the autosensing wakes up when the Amplifier sees a signal present. After the signal shuts down, it usually takes 15 minutes before the Amp goes into standby mode waiting for the next audio to be present. Note, Make sure you use a well-built 3.5mm to RCA Adaptor cable. The AMP300 and AP850 are a great match. Also, the AMP300 is 4 ohms stable so you can add a second pair of AP850 to Speaker B and both (speaker A and B) can play at the same time.
Good choices
TL/5442