We still come back to the problem that some pickups do it and some don't. And the problem only occurs at highest gain settings.
When it squeals with the guitar connected you said it stops if you turn down the guitar volume a bit. How about if you turn down the master? Does it stop squealing with high gain settings when you turn the master down?
The idea that it is not the pickups because it does it with a cord plugged in but no guitar connected is not logical. Any high gain amp will squeal under such conditions, it is to be expected and perfectly normal.
I am not saying there is nothing wrong with your amp, only that so far there is no proof this is not normal feedback. If it is really a problem with the amp, I can't understand why it would be pickup selective.
When it squeals with the guitar connected you said it stops if you turn down the guitar volume a bit. How about if you turn down the master? Does it stop squealing with high gain settings when you turn the master down?
The idea that it is not the pickups because it does it with a cord plugged in but no guitar connected is not logical. Any high gain amp will squeal under such conditions, it is to be expected and perfectly normal.
I am not saying there is nothing wrong with your amp, only that so far there is no proof this is not normal feedback. If it is really a problem with the amp, I can't understand why it would be pickup selective.