iPhone music playback with Y cable in MINI

Hello,l I’ve already searched the forum, but found no answer to my question. Although I bought the original Y-cable from BMW, the sound via Bluetooth is much better. Does anyone know this problem? Does anyone know if you can set something there etc.? To the info: I have the Radio Visual Boost with Navi etc. Software update to the latest update I have already made. Equilizer is on standard. Thanks and many greetings