Ok, my power supply is 12V. I've bridged both sides for two channels at 11W. Hooking up the speaker to just one side with one input, I get a pretty powerful subwoofer if the input is right. With my computer, I was able to get a powerful bass output without any kind of extra amplifier. My speaker is 4 ohms (Altec Lansing 6" subwoofer).
I used the schematic that the datasheet provided, but I don't have any way of etching PCB's, so I built it on a RS project board. It seems to work fine, except I get a loud buzzing sound when it is plugged into a device that isn't playing music. I also get a "popping" sound at a seemingly fixed interval, about once every second. This popping sound occurs even if it isn't connected to anything, I'm suspecting it's a capacitor charging and discharging, but I'm not sure.
Is there any way you can use a capacitor to filter noise out of an input signal? The datasheet recommended a 4.7uF capacitor between the input line and the chip, but did not specify how to wire the ground pin from the input. I hooked the ground from the input signal to the common ground of the circuit, but I'm not sure if that adds any noise or not.
Also, the circuit works fine with my 12V 800mA RadioShack transformer plug (well, except for the noise), but when I use the "13.5V 4A" transformer that was inside the subwoofer box (I rewired it, with the 110V AC input connected directly to the transformer using wire nuts and the 13.5V AC output going through the rectifier that was on the main board, it tested 12.5V or so on my multimeter), it plays louder with more noise and more static until it just stops playing and makes crackling sounds and then nothing (I'm thinking it's some kind of thermal overload protection).