Final had time to finish this project. I've added 47uf tantalum caps for filtering the reference voltage. The ground from the xlr cable coming in is connected to the chassis right at the input and the PSU ground connects to the chassis as well.
The unit is dead quit now and measures as good as the board did with the differential op amp it came with, so I didn't make any further changes to the grounding etc. But I'm using a high quality regulated PSU and added chokes to further filter all incoming voltages.
Here's the pcb design. If anyone is interested I'll upload silkscreens, copper layer and a bom.
Thanks for all the help!
