It's always a tradeoff between cheap and simple, verses something tough and well behaved in this PSU.
The design is bound by needing an op-amp U3 inputs that can go below GND for current-sense (or -ve rail), and an op-amp U2 with very high output voltage.
If you lower the op-amp U2's +V rail, you are also lowering the max. Vout capability of the power supply.
So we can't add a Vreg or zener there unless the loss is OK, or we reconfigure the output (transistors) stage.
Example: 27V zener, if U2 was outputting 25V, you lose (D12, R15, Q2 E-B, Q4 E-B) drops, total around 5V. so 20V max. Vout.
MC34071 around 2V lost and
TL081 over 3V lost.
So the penalty for D12 is another loss of 1V max. output. and probably Q2 E-B resistor for speed. C9 has to connect across pin 2-6 on the op-amp.
My small opinion is audioguru's circuit looks good but I would add protection diode (O/P trans C-E), and current limiting resistors for op-amp and Q2, and need an undervoltage lockout so there is no overshoot possible during power up/down.
To troubleshoot one of these PSU's just measure the IC voltages and post them, as well as some at the transistors.