lab PSU based on a single power amplifier?
Here is the schematic of a typical inexpensive bench power supply (from Dave Jones' EEVBLOG): 



This is an example of a real product, a mature one, where cost has been pared to a minimum for the desired level of functionality and performance. 

Most of the functionality of the outrageously expensive OPA548 power op-amp is performed by a couple 2-cent op-amps and a few pennies worth of power transistors (Q1, Q3, U1A/B). Note the two relays which control the AC input to BR1 in order to keep the linear power supply dissipation within reason. 

Note also the substantial output filtering of C6||C7. The OPA548 is not stable with more than about 1000pF on the output, so there would have be additional components added to prevent it from oscillating if you want the AC output impedance to be low. The OPA548 also has no reference, so it has to be added. And no way to control a fan, so a temperature sensor is required unless you want the fan screaming all the time. This power supply goes all the way down to 0V (not 120mV). Etc.