IPhone 6s Plus volume is very low, how can I fix this?

So it's not the headphones as they work perfectly fine on every other device I have. I have the volume limit off, I have the EQ on 'late night' which is the loudest yet the headphones are very low in comparison to my older iPhones and my iPad. If I were to compare it, I'd say my phone on the loudest is the equivalent of just over half on an iPad. So have you got any advice?

- Thank you

You can buy headphones that have volume that can be turned up more than the phones volume
For example if I had my iphone on full volume and was listening through the headphones which were only on half volume i could turn up the volume of the headphones to make it louder even when my phone is at full volume already

My volume on my 6S is very low. Volume is at max on the side bar. Volume shows at maximum during phone calls. I noticed it started doing this after using Bluetooth ear buds. Once I disconnected the Bluetooth it started having the issue. Sounds great with earbuds or on speaker.