Nove having a line6 repair center near enough, I didn't have too many options.
I thought this could be just a bad solder (happens quite often in electronics).
I eventually decided to take the amp apart (that's only six screws) and I reheated all solder bumps (adding a little solder sometimes) on the power board and the electrochemical capacitors (throughboard) of the preamp/effect board.
Took me overall 1/2h and everything is now working fine after re-assembling the amp! No hiss buildup and no cracking/pop noises anymore :-) .
I'm not saying everyone should or can open their amp and get to reapair it.But in my case, one of the solders was obviously weak and the resulting bad connection was creating the problem.
Hope this helps,