This is a bit high-end-pricey but works very well:

In-Line Water Softener With Fittings - Cleaning Tools - Car Washing - Car Care - Griot's Garage

Sorry for the late reply

I know for sure that the Adam's great white towel is awesome as those are what i use now. they are a little pricey but if you wait for one of Adam's daily specials you can pick one up for free as a bonus.

Edit: The towel is usually paired with a bottle of their soap which is awesome, i liked the soap you are using as it is probably one of the best shelf soaps in my opinion but i have found the Adam's soap to be superior to it.

The other is the Uber drying towel found on Detailers domain. They are less expensive and I hear they do a good job but i have no experience with them so I cannot say how good of a job that is.

Sometimes it comes down to the wax you use as well I dont think you said what type you use but it does make a difference in the long run. I am not going to get into another wax thread but just look up Zaino or Adams for the details and make a informed decision, just a side note I use Zaino, have a black car and do not have the problems you describe and probably have the worst water I have ever seen here in tampa.

The last wax I used was Menzerna. Im thinking about checking out Zaino. I want a wax that I have to apply no more than once a month haha. Obviously good quality too.