So I got a <$3 relay module with a 5V trigger, sacrificed some old cables (power and USB), put it all in a small box with some other parts I had laying around in the garage and now the subwoofer uses 0W when the receiver is off. What a difference:
I am sure you can tell from the graph what day it got installed (This is the subwoofer trace from the HS300). 
Works well!.