Q: Headphone adapter on IPhone 7 sound quality issue

I recently got an iPhone 7. I plugged my usual headphones into the supplied adapter to listen to iTunes. After getting used to the awkwardness of the adapter I quickly began to notice that sound quality through the adapter is far the quality of my iPhone 6.

The sound is very compressed and flat, similar to FM radio (but not no where near as bad). The bass is there but is no longer crisp and punchy. The treble is clear, though, lacking in dynamics, it sounds very thin. I plugged them back into my wifes iPhone 5 and the difference is like night and day. The iPhone 5 outshines it in every respect. Rock sounds punchy again like before on iPhone 6.

*My question, is there something in the settings that I may have set wrong that I can fix to make the audio sound as good as my iPhone 6 did? The only setting I can find is the equaliser and I have had no luck.

I am currently using my iPod classic until I can work this out.