How do I lower the amps without tampering with the voltage?
Electrical devices fed from a constant voltage source such as the mains or, in your case, a battery draw the current required to operate. This is why plugging your desk lamp into the national grid which can supply gigawatts of power does not vapourise your desk.

If your amplifier requires an amp or so to run that's all it will draw from a 12 V supply no matter what the supply is capable of (providing it can supply at least an amp).

To help further, you can add up the amp-hour (Ah) ratings of your batteries and calculate the run time. 

$$ h = \frac {Ah}{A} $$

so if you had two 8 Ah, 12 V batteries in parallel and your amplifier draws 0.9 A at reasonable volume your run time is

$$ \frac {8 + 8}{0.9} = 17 \; hours $$ approximately.

Put a 2 A fuse into your circuit to protect the batteries and wiring in the event of a short circuit.