LM1875 bridged amplifier at 12v?
Generally speaking, if you are operating the device outside of what it is specified for in the datasheet, you should not expect it to perform well - or even at all. The schematic that you included is a direct copy of the single-supply "Typical Application" in the datasheet, so you're on the right track, but in the end, you would need to find a source of at least 16V.

I'm not sure what your exact specs are, but you could consider using a device like the TPA1517, as it looks to be within your output power range and input voltage. There are others to choose from, of course, but I picked one with a more DIY-friendly package (this one's PDIP).

Disclaimer: I work for TI, but this just my opinion/suggestion and isn't an official position.