I'd be more tempted to add switching regulator to each, maybe a couple:
3.3V, and 5V.
You've found those inexpensive Chinese regulators Lefty? Put a couple of those on the output.
Well he mentioned 5 amp capacity on his basic DC unregulated supply so I assumed he wanted full rating value for any value voltage he adjusted to. But your right, switching DC to DC regulator(s) would be another viable alternative.
But the fact that his DC PS has a variac adjustable input he can pre-adjust the input voltage to the regulator to be just a couple of volts above the adjusted regulated output voltage and run the regulator in a pretty efficient manner. Variacs are great (if not large and heavy!), I got a big 10 amp unit in the garage.