Weiland -
Coupla things. Tubes are not involved in the switching of the amp. That's the case for ANY Boogie and most switching tube amplifiers in general. No worries there.
As for transformers, they are involved in the switching in that they ultimately supply the voltage for the components that DO handle the switching - relays and other components downstream from the transformers. It is highly unlikely that a transformer is a cause for switching not occurring.
In our experience, sometimes with switch itself on the footswitch goes bad. More rare but not impossible. Other times, the jack on the footswitch may go bad, get stepped on and bent which damages the jack, or any number of other circumstances that stop the switching from happening properly.
But more times than any, the cable goes bad. It may be that it gets a heavy cab rolled over it or dropped on it and the actual cable gets damaged.
But more, often than not, its the one of the plug ends that is the cause of switching not to work. You can unscrew the housing of each cable end and inspect the wires that are soldered to the plug end and make sure the solder joints are solid there. It's also good to inspect and make sure there is a non-metal sleeve that insulates the plug's housing from contacting the soldered plug ends inside the housing.
Obviously, the fastest way to test the cable is to try another and in the case of the Stiletto, it's a stereo cable and it's important to make sure you use a stereo cable to test both functions. If you only have a mono cable to use to test/experiment quickly, it will still provide channel switching but the Solo function will always be ON - so this could work for you as a quick test.
Hope this helps and let us know if you found the problem.
AB