The "Ultra" circuit simply lifts the tone control circuit (V2b) out of the signal path.
My feeling is that the tone control is mitigating the hiss.
If that is the case then the issue (if any) is in V1a/b and or V2a.
My feeling is that the tone control is mitigating the hiss.
If that is the case then the issue (if any) is in V1a/b and or V2a.