When speakers are connected to the CXA61/81, it is not possible to mute them whilst still sending an audio signal via the Pre-out. They run simultaneously. Mute and volume attenuation affect the main speaker output and Pre out in the same way.
This is especially important for those who wish to use a headphone amplifier as you cannot mute the speakers and send an audio signal via the pre-out. It would also be muted.