What would you expect this to do Rob?
I imagined nothing (all things being well) besides dropping the diode fv from the pre-amp supply but adding it as shown caused the amp to oscillate independent of the volume control and made an almighty racket until the power was cut.
You might need to add the 100uF across the power rail on the preamp side. You can also change the diode to a 100ohm resistor (no diode at all). The resistor is better as you can keep increasing the value to reduce the preamp supply changes.
The idea is to prevent or at least reduce motorboating.
Suppose you have a preamp with a 12V supply and the transistor collectors a biased mid-supply to 6V. Now you change the supply to 10V. The collectors will bias to a lower voltage say 5V (just for arguments sake). Now imagine continually and rapidly adjusting the power back and forth between 10V and 12V the change in the bias point on the transistor collectors looks like signal. That signal will be amplified by the power amp.
When the power amp clips the regulator current-limits and that causes the output voltage of the regulator to drop. The drop in supply changes the biasing of the transistor preamp. That drop in supply looks like a signal. The power amp amplifies the signal but the signal causes the supply to drop or rises. So you end up with a feedback loop where the power amp feeds back signal to the preamp via the power supply and it oscillates.
By putting a diode and cap, or a resistor and cap on the power supply the power supply changes are reduced. Hopefully to the point where it cannot sustain oscillations.
The ultimate decoupling of the power supplies is to have a separate regulator for the preamp. *But* that's only true if the input supply voltage does not drop too low when the power amp clips. If the input power rail drops too much the preamp regulator will not regulate. You will get back the same problem you had before where the preamp power rail is dropping because of what is happening at the power amp.
Anyway, the fact you are seeing a change in behaviour is a good indication we are on the right track.