Bass guitar strings lose their tone brightness after heavy use and when that happens they need to be replaced. One of the reasons why bass strings sound dull at some point is that grime and sweat accumulate in the winding, which affects the sound they produce.
An old trick that will help you get some of their brightness back and thus use them again is to boil them. What boiling does is to bring out the grime that you can’t reach when cleaning them the usual way. Once you’ve boiled them and the unreachable dirt has come out, they will have a brighter sound.
It’s true that they won’t sound like new but boiling will trigger some sort of improvement as far as the sound produced is concerned. Boiling might not work with all string types and careful attention should be paid when carrying out this task as it is not risk-free if performed incorrectly.