The 3 filters you changed aren't going to do anything for the tds. The 2 you didn't change are the ones that lower tds. The issue you most likely have is the fact the first ro membrane is feeding the second membrane. This means the "bad" water out of the first unit goes into the second unit, so it much higher tds than the first membrane gets. There is no way for us to know if the 15 out you are seeing is ok. We would need to know the tds of the water that is flowing into the second membrane. Also, you should be testing the tds of the water coming out into a clean cup, not your storage container.