The only idea I have is to swap out the offending XPS3.
Jeff, that's probably what I'll do, but doing so would be admitting defeat! I was hoping to at least see some wild theory about the AGC not working properly in that XPS3 and it seeing transmissions with no attenuation from its own circuit as distorted from an overly compensatory signal gain, even causing it to reject the entire command despite receiving half of it more cleanly from the XTB-IIR. Okay, that's really a reach!
If I replace it, I will probably use one of these this time:
One Load Appliance Micro Module [X10-2267]
http://x10-hk.com/store/product_info.php?cPath=21_29&products_id=45I actually installed the XPS3 inside the lighted sign enclosure and wired its local switch to one on the outside of the enclosure because I wanted local and remote control of it and didn't want to install a wall box just for that purpose. At the time, I couldn't find any better options, and it's been working fine for 5 years. The micro module would work perfectly here, they take up a lot less space, and I've had better reliability out of these so far than my XPS3s. They are designed for 220V but seem to work just fine with 110V. The only problem I've noticed is that they are supposed to switch back on after a power loss if they were on before, and instead, mine need an OFF then an ON command after a power loss. They may not do this on 220V, but I haven't tested that.
There is also a dual load version:
Two Load Appliance Micro Module [X10-2268H]
http://x10-hk.com/store/product_info.php?cPath=21_29&products_id=71I should also point out that, compared to two XPFM modules, the dual load micro module is much smaller, cheaper, quieter, and in my experience, more reliable. This is the kind of thing I would like to see Authinx produce. I would even pay more for it if it were a US-made product. Do you hear that, Authinx? For the interim, hopefully these will be useful to someone else also.