I bought the Line 6 Spider Jam 18months ago and it has performed really well until now. During power up there is a loud popping noise and the output for the usual settings is now much quieter than previous. Also, if I now turn the master volume above 1/2 way, this causes one of two symtoms which are set off by deeper notes (either by the guitar or the preloaded backing tracks); the popping will start again or a long continuous feedback style noise. I have checked for loose components and connectors and reset to factory settings without any joy.  Any help much appreciated ...