Most likely a wiring error or a bad solder joint, on the board or the output transformer terminals.
The transformer terminals are embossed on the bobbin - you can't see the bottom ones, but they are numbered in the same direction as the top ones (i.e. NOT in a circle!). The terminal numbers are silkscreened on the PC board so that should be easy to check. If those are OK, it's worth the effort to dismount the PC board and inspect all the solder joints on the board - if you are handy with a soldering iron, you probably know what a good joint looks like, no blobs just smoothly wetted surfaces.
Did you get the manuals (one for the amp, another for the switch board)?