Amp popping when put on standby

[jcbranam]

I have a HRDx and when I put it on standby after the amp is nice and warm it makes a loud popping sound. If I go straight from on to off without putting it on standby it doesn't make any noise. When I am warming the amp up and go from standby to on there is no popping noise either. Just from on to standby before I shut off. Any ideas what could be causing this?

[teleamp]

The pop is normal, there is no need to go to standby to power down, just turn it off.