Stabie:

I have had problems with external electronic crossovers going into oscillation for an instant as the power is turned off, causing noise to be passed to the tweeters. I have dealt with it in two ways: I use a capacitor in line with the tweeter that has a cutoff of 1/2 the crossover frequency. It is cheap insurance in case an interconnect gets pulled, and has saved my tweeter on more than one occasion. The other is a timed delay turnoff device that turns off the crossover (or in your case any offending device) about 30 seconds after the power amps have shut off. This was constructed by taking a 24vdc coil relay and constructing a power supply with large amounts of storage capacitance so that the relay will remain pulled in for the required time after power shut-off. The relay controls the main AC to the crossover. This works like a charm.