HT and heater - should join at output of psu board, attach this to central ground point close to mains connector. Now it looks like you took this wire from input tube instead of psu output, although some people ground at the input. I never do this because phantom is always "a bit wrong". Phantom should be separated from this ground, consider it as independent from audio and mains ground. - from pin 1 goes to phantom switch and from there to phantom's psu board -.
Pot between 1st and 2nd tube should be shielded and grounded properly, i will recommend some reading about it.
Looks like secondary of input transformer is a little too long and crosses heaters, probably HT too, about 1:10-15 ratio is ok, no need to go lower. I would move it closer to input tube with very short wire, put heater wiring behind the tubes and don't have HT anywhere near it. DI input could be problematic with this kind of implementation, relay is very handy in situations like this if you can't make more compact circuit. That means following schematic, don't cross wires or even run others along them. Make sure that OEP doesn't touch metal plate with pins, input transfomer needs mumetal can grounded very close to transformer, shield between prim. and sec. winding goes to signal ground.
As another member said, mains transformer could also be cause of troubles, there is enough space to move it around. Chasis like this is big enough to make really quiet preamp.
Read Ruffrecord's (Ian Bell, Custom tube consoles) "Grounding 101" and other things about this matter on his website, Jensen and Rane papers will help too.
Seems you have grounding problem, power transformer close to input and output tansformers, HT has - wire very far from it ( is it even connected directly to - pole of it), etc. For crude test without microphone you could connect signal from computer or cd player to pins 2 and 3 of input xlr (short pins 1 to 3 at the source because signal is unbal.), use a pad to not overdrive it. Maybe looks like a lot of work, but big chasis like this is advantage if you are not very experienced in building tube gear. I'm sure your work will be rewarded with great sound and low noise.
