Some subwoofers and amps have a circuit that turns them off and on
base on the presence of input signal. I'd like to either find a
device, or a circuit that will accompllish this. I have a subwoofer
that takes a line level input (amp sub out) that I'd like to switch
on and off with the amp. Please don't suggest the switched outlet on
the amp, this particular model has no such outlet.

TIA