With one channel at a time 'connected', does the sound still comes out intermittently?
BTW, the 'Sleep' circuit is on the i/p board; I would start by verifying the 'soft start' section instead, starting with C161 (Timing cap). THis circuit is part of the initial (power up) fault detection. This is where the power Enabling circuit starts. Look on the main board schematic for the crowbar circuit and move up from there. Find out if the Ready (should be a logic high) signal (falling in and out) follows the output symptom. This is a pretty complicated circuit! Go slowly and verify your signals.
The APM/Display board is available from an authorized Crown service center. The APM board is located in the front panel. I can't reliable say what could have caused this symptom. Though, two units that have the same behavior? Perhaps, if at all possible, try to confirm with another XLS owner, or with the service center that the APM board is really defective. Hopefully, it may be an issue with the LV psu!
I received this explanation :
" The 21/NN error code is display on the LCD not the thermal LEDS.
The 21 error code/NN error code is a DSP Boot failure, the NN = 99 means it has occurred 99 times which the max the error buffer will register.
The APM/Display board must be replaced to correct this error." (TY Rodney, for this explanation!)