If the transformer is bad, it is bad because several other components are shorted and caused the transformer to GO bad.
It's NOT just the transformer, it's NOT just what you can "SEE." There is a CAUSE for the transformer failing! You are looking at the symptom, not the CAUSE.
If you do not have test equipment, soldering equipment and the knowledge to use all of the above....you should not attempt to repair it. You can make the problem much worse or shock yourself.
Buying a new sub is almost ALWAYS cheaper that attempting to repair one without the necessary knowledge, tools, or experience.