oh, thank you very much. so i will definately allways switch it fully off. thanks a lot!No, you were clear. I think you misunderstood my post. The SMPS in the device needs a separate standby PSU to be put into actual standby mode (0.4 W). If the amp doesn't have that - and the pictures on the Audiophonics page don't show it - the SMPS will always be idle and consume 8 W, even if you turn it "off" with the front button. You need to fully switch it off with the rear (mechanical) switch to stop the idle drain, as far as I can see. The stereo variant (HPA-S400ET) seems to have that extra standby PSU, by the way.
Sorry sou're having trouble with one channel. I'm sure Audiophonics can help you resolve that.