Update:
It seems that the culprit is the sub-regulator board for the preamps.
Disconnecting this board and connecting the rails directly to the PSU puts it to work, with what seems to be a normal noise floor.
On the sub-regulator board, for each harness it reads GRD 10V -10V 10V -2V. This last value seems off to me. When connected directly to the PSU, the last value is -10V.
Does anyone know what's the actual function for this sub-regulator board? Lowering the noise floor for the preamps is my guess but I don't really know.
Will I be damaging the preamp cards running the power directly for the PSU without passing through this sub-regulator board?
To my knowledge, information about this is non-existent on any of Digi's websites.
Btw, the regulators on the sub-regulator board are: 1x LT1963A and 2x LT1175.
LT1963A:
http://cds.linear.com/docs/Datasheet/1963afe.pdf
LT1175:
http://cds.linear.com/docs/Datasheet/1175ff.pdf