Having recently built a 0-18V 0-1A bench supply, and plans to build a 0-30V 0-3A, here's my five cents on the matter.
First off, powering the opamps from the same supply as the raw supply voltage is fraught with issues. The best way to alleviate these problems is dont do that - most linear bench supplies have a separate supply voltages for the opamp, with the midpoint tap connected to the main regulator's output. This is a bit of a headf*** at first, but it works extremely well - i believe it was HP/Harrison Labs who came up with this "Regulator within a regulator" topology.
This may sound complicated because you need the extra transformer tappings, but you can always use a second small transformer, it doesnt need to be powerful, so some tiny 5VA thing from an old digital bedside clock works very well. In fact that is exactly what I used when I was prototyping my supply... and I could have built the end unit like that too, I just happened to find a suitable transformer core that I could re-wind with custom secondary taps.
Tap switching is well worthwhile doing to vastly cut regulator dissipation. Use a 2x18V transformer for 0-30V (remember it sags vastly under load so you need quite a bit of overhead), connect the windings so you have 0-18-36, and you can easily arrange a circuit which can switch between the 18V and 36V AC taps.
I've attached the main schematic of my power supply. It works well, doesnt oscillate or do stupid things under load, and uses only bog standard TL082 opamps. I haven't shown the front panel board, but it is only metering and display. The controls are 10K linear pots wired to CN2 with their top wired to VREF, the bottom wired to AGND, and the wipers to V_FINE, V_COARSE, and I_CONT respectively.