Whether the amp uses more power when playing or idling is dependent on the technology of the amp. A typical "Class A" (not to be confused with a Stereophile "A" rated) amp will use as much power idling as playing at low to medium volumes. If the amp has a standby switch, the standby mode should reduce power consumption to a very small amount. If the amp has an on/off switch, that should reduce the power consumption to nothing. If this info isn't in the manual or on the back of the unit, you'll have to get it from the mfgr, or measure it as mentioned above. To throw in a little more technicality, you'd have to use a Watt meter to get the most accurate measurement (due to phase angle), but measuring the current (amps) will get you in the ballpark.
