+1 to other suggestions like star single point wiring for the grounding. Especially look at the grounding for the amp input side as you can pick up racket here easily.

To filter ~300 Hz you want a large-ish inductance, a ferrite bead wont do it, a ferrite toroid might help

Maybe try an automotive noise filter for stereo eg https://www.amazon.com/Mr-Ho-Amplifier-Suppressor-Stereo-Isolator/dp/B08213FMPN or many similar