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The duty solenoids get the 50Hz duty cycle. With ignition on and engine off you can hear them buzz as you change throttle position. You probably need to put the car into garage and open windows. I'm not sure which ones you can hear.
Yes it is just 12volts. Polarity is positive of course. The other end of the solenoid is connected to the chassis/ground.
A fluid passage blocked with a small piece of clutch material or metal debris would explain your symptoms. Sorry I don't hand the manual at hand, but typically how Subaru does valve bodies is they have a small calibrated passage from the line pressure to the chamber connected to the solenoid. If the solenoid is open, it drains the pressure in the chamber to the sump and allows continuous fluid flow around the solenoid and through the calibrated opening. if the solenoid is closed, the pressure builds up. My point is that a grain of sand is enough to block the calibrated passage and once it is there, it is likely to stay there.
Where I'm getting at, electrical connections and wiring are much easier and more pleasant to play with, but the best chances to get it fixed is with the hard to reach and messy hydrolics.
It may turn out that cleaning the extension valve body is all you have to do. However you may need new gaskets that nobody stocks, don't expect it to be one day job.
In my own case I had solenoid A go at 154k miles - It would jump wide open and stay wide open. An "autopsy" did not show anything broken inside. The moving parts had obvious wear, but nothing broken in half...
Do get it fixed ASAP - don't drive without AWD. You risk too much load on the front diff (can break) and more damage to the transfer clutch/bearings in transfer case due to oil starvation - no transfer pressure means no lubrication there.
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