Random Amp questions

Thought I'd start a thread for this. I am into another amplifier design. This time its on strip board. And I'm likely to have many more odd questions.

It's basic topology is a diamond front end, beta enhanced VAS, HEC output stage with one pair (currently) of vertical MOSFETs. Compensation is TMC.

First question is whether it is possible that having too low ULGF can cause instabilities in itself? Or having too much closed loop gain? Using Tian probe I have a main loop ULGF of 600Khz and a closed loop gain of 30dB. In this configuration if I have the output stage biased above 0mA or voltage rails above 35v it breaks into oscillation. If I cut the feedback shunt resistor it stops oscillating and can have OPS bias and 60v rails. If I decrease the closed loop gain by increasing the feedback shunt resistor then it is stable again but the ULGF increases (800Khz). Don't understand this I thought having a low ULGF would be less problematic.

Second question is that with TMC the loop gain plot has a shelving just above the ULGF which kills Gain Margin. Switch to TPC and the shelving disappears and gain margin is massively improved. Is TMC incompatible with the HEC output stage?

Thank you

Paul