BTW, did you follow Neve protocol with regard to the grounding and shielding? If not, this could also be a source of noise.
Basically, every feed to and from components (input transformer secondary to PCB, PCB to gain switch and back, PCB to fader pot and back, PCB to LO1166, LO1166 to output connector, etc...) should use shielded cable with the shield connected to audio ground (referred to as "B-" in all class-A Neve drawings).
As for grounding, the power supply's 0v should have one wire connecting it to the earth/ground pin of the AC wall connector and a second wire also from the PSU 0v to audio ground of the PCB's, input transformer secondary, etc. (just look at 1073 schematic to see what I mean).
Then, making sure that all sides of the chassis are electrically connected (Dremmel off some powdercoat on finished chassis to allow electrical connection all the way through the case), connect the chassis with one wire directly to the earth/ground pin of the AC wall plug. In this manner, the connection between audio ground/B- and earth/chassis ground occurs only at the single point of the AC ground plug.
Lastly, make sure to tie audio transformer cores to chassis, not audio ground/B-.
Oh yeah, and do NOT connect the B- and chassis pins of the modules together (pins E and V, respectively, on a 1073-type module). The connection between these two should ONLY occur at the AC wall plug ground pin. Lots of people argue with the need for connecting the two types of ground at the power inlet only, but trust me, if you are careful to do this, your noise floor will drop dramatically. Most people just jumper pins E and V on old channel modules and then connect that to both chassis and AC ground and figure it is the same as I describe above, but it isn't.
Coupled with zealous shielding, you really shouldn't have any noticable noise issues (unless the BA437's in the 3126's really are as questionable as I've heard).
Your two BA283AV circuits should be dead quiet though.
Please let me know if you have a chance to compare the 437 against 438's. Very interested.
Peace,
JC