One last thing to figure out is the pop I get when turning it off and on.
Are you using a delay on the on/off control, or just letting the amp come on right away when the power supply ramps up? The on/off control is supposed to be pop-less, so putting a power on delay on that control to delay amp turn on until the power supply is fully stable would likely solve the pop problem.
The Hypex SMPS1200 power supply has an amp enable line for that purpose. Are you using that supply, with the amp enable connected between supply and amp modules?
Are you just removing AC power with a switch, or using the power supply standby control to turn power on and off (assuming you are using a Hypex supply)?
The SMPS PSU's ground to chassis via PCB screw hole
OK, verify that there is actually a low resistance connection from the PS GND output pin to some other point on the chassis. Ideally you would be able to control the grounding point, but as long as your chassis actually is conductive should be OK. It looks like the PS would screw to the lower panel of the chassis, so pay special attention to checking from the PS GND to the rear panel, since it is easy to get high resistance connections when you have multiple panels connected together.
amp modules ground via heat sink to chassis
I'll take your word on that, it is not mentioned in the datasheet. In cases like that you have to just hope the layout designer did the job properly and doesn't let interference current cross the audio circuitry, or have a heat sink which is isolated from chassis. Those amps don't put out much heat, so it is easier to have an isolated heat sink compared to a traditional design which might need substantial back panel space for heat sink fins.
I always thought best practice would be to star ground these.
It may depend on how you define your star. The goal is that interference currents have a path to travel which does not include the audio circuitry. You need to have the audio circuitry referenced to the chassis at one point, not multiple points for that to work correctly. That is a logical star, but not necessarily a physical star (i.e. a star ground doesn't necessarily involve long wires all going to one point, it could just involve single conductive path to one point, e.g. short pin 1 to chassis connection which then use the chassis as an "arm" of the star).
This is how the referenced AES48 standard draws the recommendation:
So potentially should I also be looking at my monitor controller as creating the issue!
Output connectors can have pin 1 problems as well. Start by getting your amp wiring correct, then evaluate whether any changes are needed to your monitor controller. Same rules apply everywhere, so you can pop open the case and tell in a couple of minutes if it has a potential problem or not.