I have a Fishman Loudbox Mini that has recently started playing up.  The signal output level is much lower than it should be and it produces a lot of 'sh' whenever anything is plugged in (even just a jack plug without a cable attached).

Through reading discussions here and on another forum, I gather that a full schematic is difficult or impossible to come by.  Given my description of the fault, can anyone suggest any possible causes and how I can test it to pinpoint it?  Has anyone else had the same experience with the Loudbox Mini?

Many thanks.