Thanks for this. Further investigations reveals a number of smart sockets, and cheaper than the first one I linked to. This is important because if the aim is to cut down on unwanted energy use, rather than just for the fun of tinkering, then the less the initial outlay the better.
How to auto switch your power amp with IFTTT looked promising (I found a smart socket on Amazon that supports IFTTT):
But, it is not clear what I would have to do to make this work. Is there an extension I can install and play with?