Navi 900

I have a vauxhal Insignia 2014l car with with a Navi 900 in it , which is a sat nav radio etc, now I have put a new amp and speakers to upgrade the sound , and for some reason now I have lost 50 percent of my dab reception, the car aerial is a shark fin type , the fm and am reception are unaffected thou, the reception was fine before i put the amp in, the dab is not one of those stuck on the window types either .

If anyone has any ideas could they help as im at a loss .

Thanx