[julie_m]

If the idle current is set too high, then the battery life will be reduced. (The power dissipation in the output transistors will also be increased; but not by as much as it will increase anyway under signal, unless you set it outrageously high.) If it is set to low, crossover distortion will be audible. This will sound like the speaker voice coil is binding.

Think of the output transistors as a potential divider. If the "top" one is conducting, the output will be driven towards the positive rail; if the "bottom" one is conducting, the output will be driven towards the negative rail. Now, a transistor does not begin to conduct until it has a certain voltage across the B-E junction (about 0.7V for Silicon, 0.2V for Germanium). So the output transistors have to be biased so that the no-signal operating point is somewhere within the linear region of each transistor; if you superimpose the transfer curves of the output transistors, they cross over at the no-signal point. Both transistors will have to be conducting a little bit, meaning the current through the output stage with no signal will be greater than zero. The harder "on" the transistors are biased, the greater the current under no signal conditions; and this current can be measured as a useful indication of the biasing conditions.

If the biasing is insufficient and there is an input range where neither transistor is conducting, the speaker cone will not move until one or the other transistor begins conducting and the amplifier moves out of the dead zone. This is crossover distortion. It tends to manifest less at higher volume, since the signal is spending more of its time out of the dead zone. Of course, if you turn the volume up too high, the signal will become distorted again as clipping sets in .....

It's easy to see crossover distortion on an oscilloscope; but if you haven't got one, you can set the quiescent current approximately by ear. Find a good, strong station and turn the volume down until you have to start straining to hear it. Adjusting the current downwards until distortion is heard. Then tweak it up just a little, for luck. (You will need to tune away from your station and set the volume to minimum actually to read the current, of course, since it will increase when sound is being produced from the speaker.)

It is probably best to set the current using slightly-spent batteries, but then check it isn't too high with fresh batteries or a suitable PSU. (If you have an adjustable bench power supply, this can be turned down to simulate spent batteries.)