Bridging Discrete (BJT) Amp Channels
One caution with the bridge-tied load....

Your load should be considered 8 ohms in series with a 1000uF capacitor. This will be a polarized electrolytic capacitor, whose polarity must be carefully observed. (Reverse DC voltage across such a capacitor spells disaster).

It will be inevitable that one of your two power stages will output a DC voltage higher than the other. But which one? You could measure them, and then place the electrolytic capacitor polarity appropriately.

For the single-ended case where only one power stage is used, there is no polarity problem - the negative end of the capacitor faces the negative end of the USB supply.