My fretless was made using a cheap plywood body, works fine , sounds great. I even made a prototype out of a sheet of 18mm ply, even recorded with it.If you've they tried a plywood bass and it sounded crap the chances are that it was a crappy bass all round.
As I see it (and hear it) most materials for making bass bodies don't make an enormous difference, yes, some will give greater sustain, brilliance etc, yes some will give a duller tone, less sustain etc, but 90% of possible things you could use do the job perfectly well and I include plywood in that 90%.