Test the rectifier with the diode setting on the meter. Should read about .7V one way between each of the ~ and +, and between the - and ~ (red - black probes), then should read "open" on the reverse (black - red probes).
Problem is you might have a transformer winding making it read 0V, and the big filter caps can throw it off, too. Really need to disconnect that bridge first.
More common than the bridge is a blown driver. These often fail short. Disconnect the speaker, use the same diode check, and see if you get a short to ground or to the supply voltage on the speaker output.