Unless both amps have the same problem, I would consider looking at how much variation you see in the line voltage , you might be better off getting an autotransformer from the outlet to your amp  - this will regulate the output voltage to your amps .

only way I could think of checking this would be to use an autoranging multimeter with a high memory function - this would capture the peak voltage. Really difficult to address a problem in 2 different amps , especially if the same problem occurs. Unless of course they are identical amps and both are faulty

..dB

Is it always around the same time (evening and morning) ?

It is possible the tones that are used to switch from day to evening power or used to switch on/off streetlighting is audible in transformers.
It usually consists of a burst of tone(s) with or without an interval and changing frequency or not.
Some transformers make those tones audible but they should not be audible in the music unless common mode rejection of the mains is 'off' for some reason.
Harmless but annoying.
Since the tones used are in the audible range (between 200 and 500Hz) they are impossible to filter out.

perhaps I am barking up the wrong tree and it does depend on the electricity grid and local circumstances.

i used to get TX buzzing from a Naim amp only in the evening after 10pm.  Naim told me that DC riding on the house AC was the cause.