This is a problem I have seen a few times before when the powerlink socket is used on the passive amplifier, its a wiring fault on the powerlink socket where if my memory serves me correctly pins 1 and 4 are reversed which are yellow and grey I think, but I will need to check the colours as my memory is not what it was in my younger days.
If you are happy to resolve this problem yourself, then just reverse the wires defined above will fix the problem. The signal is there to shut down the speakers before the TV actually switches off.