I would simply disconnect the main power from the main amp stage, put bleeder resistors across the main filter capacitors (if there aren't any already) and use the convenient pre-amp out jacks on the rear panel.
As an aside - I repair this sort of stuff all the time. My only concern in fixing it would be that this type of output transistor is an obsolete package and it might be necessary to hack in a different physical package. Ugliness potentially ensues.