No preamp is going to be stable during power up and power down. Internally voltages are changing rapidly and operating conditions for the preamp stages are not established. I would expect some output transients.
It's not your cables. You have to ensure that your power amp is the last thing turned on and the first thing turned off. The trigger does not enforce this.
It's not your cables. You have to ensure that your power amp is the last thing turned on and the first thing turned off. The trigger does not enforce this.