I am getting constant fuzz on one of our SVT heads at all volume levels. All of the bias levels on all of the tubes are correct and the tubes are all new. To me that would indicate that all the resistors are within tolerance. This is an original SVT with the 6550 power tubes from the early seventies. All of the board components are original. Would coupling caps that are out of tolerance cause this? Someone suggested that the output tranny is shot. Could that be the case.

Try swapping the tubes from another SVT. Even new tubes can be faulty. Also, is the head hooked up to a single cab? You may try it with another cab. Maybe there is a problem with the cab.