Powering AC Appliance with DC
If it's just an audio amplifier it should work without damage at reduced voltage. You can try it at 12 to 14 V from your car battery, for example.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. AC/DC power can be used for AC/DC music.


Adding D5 will prevent backfeed into your battery when mains is on. 
If battery DC voltage is less than mains DC voltage the diodes will ensure that the unit always runs on mains when available.
Use your multimeter to measure the actual DC voltage after the bridge rectifier. That's probably your maximum battery voltage. Minimum might be half to 2/3 of that. Pick a battery voltage to suit.
If you can break into the DC line somewhere you can measure the current. Use the 10 A range with the probes in the correct sockets.


$$ Battery~life~(hours) = \frac {Battery~Ah~rating}{Amplifier~current} $$

Size battery to suit.