My active studio monitors are consuming a lot of power when they are on even when they are not playing anything, they are bi-amped so two amps per cabinet. Being not only environmental but also cost aware this needed solving. Off-the-shelf switches were ugly and expensive. So I resorted partly to Ali and partly to my local electronics shop.
The power supply I constructed using parts I had laying around (look at that expensive audio cap!), ordered the little green fuse box though, for safety. The transformer is from an old (unsafe) 12V supply and is fed through a 7812.
My Onkyo 607 didn't have line outputs to send to the active monitors, I dug into the mess of wires and circuit boards to find the line level L & R. The 'zone 2' outputs at the back were unused so I connected them to these feeds.
The audio switcher itself was a complete circuit board I bought at Ali for 5$. It featured an electret microphone so I had to convert my line level using a cap and resi