Just did a Google search for the same problem and came across this thread. Bizarrely exact same problem, exact same amp and exact same situation, bought it about six years ago and no problem until now. I was about to start dismantling and tightening as you suggested and I discovered exactly what the problem was. believe it or not it is my new laptop. I always sit and play with my laptop in front of me, when I moved it away the hiss went away, pulled it towards me and the hiss came back. This is definitely the problem, probably some sort of wireless thing I imagine but I'm not techy enough to understand all that. Don't know if this is similar to what you experienced but maybe worth considering for the future.