When the amplifier is idling, the SMPS is consuming about 10W, the amplifiers about 1.5W each. 10/13 = 77% (ok, 90% was excessive).
I dont have equipment to measure power consumption but i just tried measuring the SMPS1200a400 supply current in idle (nothing connected) and standby. The RMS current measured with a fluke 177 drops from ~95 mA to ~62 mA. Im on 230 VAC so the power factor must be quite bad at low power to meet the 9 W from the datasheet. Anyway if the power factor remains similar then the power is only dropped by 62/95 going into standby. Not as much as I expected. This may explain why early datasheets had standby power as "TBD" and completely dropped it in the later versions