If you unplug the load on the AM486. Can you hear the internal ratchet switch CLUNK turning On and turning Off?
If you can turn it On but then not Off. The load is probably making noise or is a signal sucker and the power line signal is marginal.
I would also suspect all the electronic gear in the top of the outlet making noise or are signal suckers. A filter maybe needed.
An 20 amp wire in type XPF filter. Would have to be wired in such a way. To filter the electronics but not the AM486 or it would not receive any X10 power line signals.
There are plug in filters. Like the ACT AF120 fifteen amp one. The Smathome FilterLinc is a 10 amp one and has a unfiltered pass through outlet on the front. Where a automation module can be connected. JV Digital Engineering makes both a 10 and 15 amp plug in filter.
http://jvde.us/xtb-f10.htmAs for a 40 watt load. I believe that is a hold over from the early before CFL Friendly ones where designed by 10WTI and had Local Control Sensing. Newer ones should be much better and the 40 watt load may not be a hard requirement now.
40 watts would apply for the Lamp Modules. As they have a minimum current requirement for the triac to conduct properly.
The newer CFL friendly AM486 module have almost no sensing current on the output. The most that will happen is a real small load may glow dimly.
The older ones with Local Control Sensing did have more sensing current on the output and would make small loads do strange things.