The player has both single ended and balanced outputs, so both need to work. Output jacks will remain wired to the main board.
So with his preamp (I presume you know who this is for), the two halves of the I/V stage boards (+ and - signals per) need to be within a mV of each other? Hmmm, didn't know what I was signing up for here May not be a problem as they seem rather close just checking individual offsets to ground, but that is on the bench and just each individual board.