By "amp", I meant that I don't have any information for the EA Doubler. I did find the data sheet for the 250ASX2 module, but the block diagram on there doesn't appear to show a mute switch. Which block diagram are you looking at?
I think the SMPS part of the module is working, since there is DC power going to the pre-amp boards and I get a signal from the Effects Send output.
Thanks for the help you've given me. I think this will be going to an amp tech, assuming that I can find one with the skills, experience and inclination to work on it. Most of the guys I know round here are old-school valve amp types. I'd just like to gather as much information as possible to pass on.