Kids might want to listen to their phones by using the speakers when on the road, but you forget so many other usage scenarios, where having a great speaker does make a difference. When I am at home and want to listen to a podcast, watch a YouTube video or even listen to music, many times I just do that on my phone. I can't be bothered with headphones most of the times, so the M9 does produce the best quality sound by far. I don't know how you think that the 6 plus has better sound, but I guess each to their own..
I will absolutely agree that sound quality is subjective. When it comes to home HiFi I've always preferred fabric dome tweeters to metal tweeters as I hate bright treble. I enjoy a rolled-off treble and a warm, non-fatiguing sound that I can listen to for hours. Some people prefer bright, detailed treble and some don't. Some prefer over-emphasised bass whilst others like a neutral and non-coloured sound. It's all down to personal taste and for me the 6+ sounds nicest. That doesn't mean I'm right and you're wrong.