This is a common problem when connecting two audio devices, each with its own volume control. The most common recommendation is to set the source (iPhone in your case) volume at about 90-100% and leave it there. Then, control the speaker volume with the speaker driver (the Bolt in your case).I found my iPhones Music Volume is way too low. I can jack up the volume on my phone but then my phone volume is too high or I can jack up the volume on the car making the radio volume too high. Anyone else notice this?