Q: iPhone 5S plays music very quietly in the car

I have a new iPhone 5S which plays music at a much quieter volume than the radio in the car with the stereo at the same level. My old iPhone 4S is fine and plays music [i]very[/i] loudly, but both my new and previous 5S phones had the same problem. Even if I turn the car's stereo system up to full volume neither 5S phone produce loud music. I have a new Volvo XC60 and all the phones and the car have the all the most recent available software updates. I usually use the phone on Bluetooth but I get the same problem if I switch the input source over to iPod and Aux. The phone settings have Sound Check, EQ and Volume Limit all switched off and the Volume Limit set at maximum.

Any ideas?