The TDA2002 is made for a supply voltage of 14.4V. Philips and ST Micro have more than 100 class-AB audio amplifier ICs and some work perfectly at 6V. A 6V battery drops to only 4V during its life.
The datasheet for the TDA2002 shows that its quiescent current drops with less supply voltage so it probably makes crossover distortion. It has a graph of power output vs supply voltage and its output power is very distorted (10% distortion) and only 1.4W into 4 ohms with an 8V supply. It might be only 1/2W at clipping into 4 ohms with a 6V supply.
Years ago I made a portable stereo amp with National Semi's LM2896 as a bridged amp. It was bootstrapped for low voltage loss and produced 5.5W per channel into 4 ohms with a 7.5V set of 6 Ni-Cads. The LM2896 isn't made anymore but newer ICs are similar.