[Croozer]

Are the DC conditions close to the voltages on the service sheet? From the snip of the circuit diagram I can see, it appears that there may be a -ve feedback circuit connected to the secondary of the output TX . Replacing the transformer may have reversed the phase and caused a ve+ feedback loop and instability. You could easily check this by snipping the feedback loop (ie the connection from the secondary which does not go to the speaker)
However guesswork isn't the best way to track down instability (I only suggest the above because the check is easy! If that doesnt work, I would start systematically with voltage checks.