In the past I have tried carbon resistors Colin, cant remember which brand, but were not for me, I found they made the sound mussy and returned to metal film, Morgan Jones books also say give them a miss, the Z-Foils I used are only in the first stage, as recommended by John, eight resistors, just over £130.00
but I can say now, money very well spent
. You mention 5%, R11/12, 13/14, 19/20 need to be 1% or better and measured and matched if possible. Bob