I have a 12 V 400mA supply pluged into the switched outlet of the cable box for power sensing. When I turn the cable box off the power supply still has some residual voltage that fools the XP-8 into thinking its still on for about 45 seconds. Does any one know of a simple circuit (resistor,cap,?) that I can use to drain the power supply in a few seconds.

Put a little diode on it in line.

Just curious, what will that do?

Will it cut off the voltage when it drops below 'x' volts?

It will discharge any capacitance quicker. (Stored Energy in the wall wort)

Could also use a relay.

Relay has the advantage of removing voltage and creating a dry contact. Might save the XP8 from a power surge one day (who knows). Anyways the relay will suck the transformer dry.

Use an AC transormer in the same voltage range.

AC supplies tend to diisapate far quicker.

Can the XP8 accept an ac input for the sense? I've only done 12v dc on one sense input so far. I would think it would work, but hey, what i think will work and what does actually work are sometimes mutually exclusive.