Aftermarket stereo upgrade has very low volume - is there an amp or interface I don't

[harro300exc]

Hi guys we have a 8/10 build Tiguan with a standard RCD 310 stereo fitted. The Bluetooth (calling itself VW UHF) and an ipod input all worked. I have just bought a brand new top of the line JVC double din head unit with carplay so my wife can use Siri on her iphone to make calls and send/receive texts. She really wanted to initiate siri by holding the phone button (as in her old car) but it didn't work in the Tiguan. The new system works well except the output sound level from all speakers is very low. So low that you can still talk with the head unit flat out. This makes it a dud as it is too low to hear a bluetooth coversation while driving.
It has something to do with the car because I took the whole thing out and installed it in my Pajero and it worked fine. I got steering wheel interface which all works, the harness I got plugged straight into the car and it powered up fine.
From what I have read some Tiguans had an upgraded sound system. I can't see any amps under the seat or elswhere but something is causing a the JVC unit to only give a low output. Has anyone got any ideas about what it could be?

[kamold]

It sounds like the head unit is putting out line level out instead of speaker level output.
ie it thinks it is sending the signal to an amp, not speakers. Given it worked fine in another car, it might be something which is automatically detected by the head unit...just a guess....

Is there a setting on the head unit to switch from line level to speaker level?