It sounds to me like if the volume was set too high, with close pups ...
First, I don't know what you did to change channel on the video, but it may correspond to a user programm saved with too high values, not reflected by the actual knobs positions.
Did you check the volume of the high gain channel?
(On clean channel, no problem as it seems)
Having a try with a bigger distance between amp and guitar would be a good idea too.
I would check every knob; gain, volume, master, treble, and see if they do act or not.
Then set them all to 0, to see if the problem is sill there.
If not, just operate one by one; i.e.:
Set one to ten, the others to 0, then see if there is still the problem.
Then set it to 0 and try with another one set to ten, etc
After that, just set the master on 2/3, and repeating the 0-10 test for all other knobs, etc.
Then repeating the test with both master and volume on 2/3, etc.
Then with master, voulme, gain on 2/3, etc ...)
Maybe you could try too with only one effect at a time: only amp sim at first, trying different amps ...
Did you try this channel before, without having the problem?