It's worth checking the transformer, but that seems like the less likely issue here. Transformer induced noise will generally either increase with volume (if it is being picked up on the input wires) or be constant with volume (if it being picked up past the pot). And you have pretty good distance here.
The grounding, however, does not look very good. It is difficult to see exactly what is going on, but what you need is a single ground point that everything connects to directly. I usually put two terminals somewhere, such as in the middle of the case, spaced about an inch apart. Run a thick-ish piece of wire between them, and call that your star. Connect everything - the PCBs, the jacks, the RCAs, the pot, the PS, directly to it.
For earth, connect the earth pin on the IEC to the case as close to the IEC as possible. If everything (jack, pot, RCA) is isolated from the case, then you can use a ground loop breaker to connect that point to your ground star, but if you are grounded correctly, it is probably not necessary, or useful. Instead, I'd just connect that point to your star ground, too.