It's always a good idea to remove the old protection systems from these phones since they don't age well. The drivers will easily last another 50 years. You can always install some back-to-back 451V zener diodes if you want protection for the drivers.
As for the SR-007, just run the amp for a week playing music and the imbalance should go away. It can also happen when the weather changes as humidity is a factor here.