Try plugging a signal in to the stereo link in , this should bypass the pre-amp or at least some of it.
I suspect they can be used as trolleys by resting keyboards on top as the ones I saw are on wheels.
I have seen some where the knobs have been pushed in from the top which has forced the pots (volume controls) to start to disintergrate.
Its possible to remove the board and carefully squeeze the pots back together
and bend the little tabs firmly around the pot body.
A schematic may be helpful here.
If you have no soldering gear or multimeter and if its not the above I'd take it in for a quote.