When I zoom in on the binding posts in one of your pictures it looks like you may have the plastic washers backwards. I'm wondering if you are getting a short on binding posts to the chassis. Those plastic washers should be oriented such that it prevents the binding posts from sliding around and accidentally coming into contact with the chassis. They should be oriented such that the small inner ring goes inside the hole in the chassis for the binding posts.
I don't know exactly, because I don't have the amp open in front of me, but the plastic washers should all be in the right position.
I will correct this if it is not the case.
But that shouldn't be the problem, should it?
My feeling is that it might also have something to do with the internal speaker cables.
Maybe I didn't solder them well to the binding posts.
Do you have any experience with this, could it be because of that?