Hi Rob. Thanks for your post. I'm going to assume you're talking about the BitsyX. Let me know if otherwise.
The BitsyX audio amp already includes a two-channel, bridge-tied amplifier. You can read more about the audio systems and the LM4863 amp we use in the BitsyX user manual (section 6.2, Audio).
I guess you're asking for more than the 2.2W/channel that the system can already deliver? You might try dropping the impedance of your speaker to get higher power output. If that doesn't help, you'll need to use an external amplifier.