Nice project! Is there a cheap place to buy this size PSU enclosure?
And a question, how does the mic SNR measure compared to the traditional PSU design? With the fixed bias design in the U67, the mic SNR is sensitive to the H-. The traditional PSU uses 20v AC and a 15V zener, followed by passive filtering, while your design starts with 9.5vAC, drops ~1.88v through the first two resistors and then uses the LM337 regulator. Also, what heater AC voltage does the H- start to crap out? If you drop from 9.5vAC I expect you'll hit the minimum voltage drop of the LM337. Typically, you want to design for the regulator to drop a few volts minimum so it can maintain regulation (it cannot regulate down to 0v across it), but obviously you don't have much Volts to work with starting with 9.5vAC
Also, it looks like you used a heat sink insulator. If you disconnect the heat sink pads from the ground plane, you can omit the insulator, which gives better dissipation and a lower regulator temp (but you are not dropping much so not a big deal).