Check the keyboard. Those have foam rubber buttons inside with metalized Mylar on one side. The foam has a bad habit of rotting out in those kinds of keyboards. I had to replace all of those foam buttons in mine with homemade ones. To check you will need to take the keyboard out of the Advantage and then remove all the tiny screws on the back side.
At the load system prompt, you can also try D2 to boot from the second disk. Or Ctrl-C to get into the mini monitor.
Otherwise, it sounds like a nice machine with those optional parts in it :)
David