Instead of using softener, use a little bit of white vinegar.

Since short, I do mix it in advance with water (50/50) to make it less acid (apparently better for the rubber). Don’t worry, you clean clothes will not smell vinegar! Your laundry will smell according to the smell you’ve added (home made vinegar with smell; hydrolats or essential oil). Try to avoid essential oil if possible (read my blog post on this to know why).

Update Dec 2019: lavender + (pure) vinegar… now waiting for few weeks to see (and smell) the results…