MBL 8011AM need your help ......

I bought a used pair of MBL 8011AM 2 months ago but didn't open the boxes to use until last week.

In one amp,kept powered on, after about 30 minutes of no signal input, the amp would shut down spontaneously.
Is this abnormal, or , just from a previous setup of the amp to power off automatically?

In another one, the same condition was noted, but just after only 3-5 minutes of no signal coming in.

Anyone with experience of 8011AM use or now with these under use? ( so you must know )

Thank you in advance!
I have the same pair I purchased from a dealer about a year ago. I keep them fully powered on all the time as I read this was the thing to do. I have never had them power down on there own nor do I think you can program them to do so. I am going to check the manual to be sure. When you power them on from the rear panel the switches light up green. Then you move to the front panel where you push the power button---once for stand by and twice for on.
Thank you for so valuable and so helpful information!