also iirc the amp only has an 8 ohm out, so that might be annoying, too.
I'd expect the output transformer has other taps on it but they've saved money by not fitting an impedance selector switch.
You will also save money by only having one secondary in the OT, so I expect that there aren't other taps as it looks like costs have been kept to a minimum.
Incidentally, you can also save money on the OT by not interleaving the windings, not using interwinding insulation (or even using a split chamber bobbin), using a smaller stack, using cheaper steel, using thicker laminations, stacking the laminates in pairs or greater, but without reverse engineering the transformer it will impossible to assess which (if any) corners have been cut.