Volume level control problems on iPhone 6

Here's an interesting problem related to music volume control:

I was comparing some headphones for my iPhone today, and I found that the iPhone doesn't always set the correct volume levels when starting playback of a song. I happened to find it by chance really. This is what happened:

I was testing a Beyerdynamics T70 32Ohms version, and it sounded not great. I had to pump up the volume a bit(to 50% in fact), which is not normal for a 32Ohm headset.

I then switched to a Beyerdynamics T90 250Ohms version, which sounded very underpowered. I know 250 Ohms is a lot to drive for an iPhone, but according to reviews I should be able to get decent volume levels if I cranked up to 8--100%, but the headphones sounded very underpowered. If I fast forwarded the song, the volume picked up significantly. Initially I thought this was a headset problem.

I then tried the same with a Beyerdynamics DT1770, and I found that too underpowered when I started a song, but when fast forwarding the song would significantly boost the volume levels, to the point that I had to reduce it from 100% to 65-75% of Max. Skipping to the next song after the volume levels had restored would keep the volume levels steady, but if I stopped completely, and started a new song selected from the list (or the same song for that matter), the volume level would drop significantly again. I actually have to move the fast forward slider of the song before it restores volume levels.

Initially I thought this problem was due to driving 250Ohm headphones, but I reproduced this with normal In Ear Monitors (I tried Ibasso Operius, and RCA T20s, and Klipsch k11i). The same results:

- Starting a song always gives low volume levels.

- Low volume levels are only restored to higher levels if I fast forward the song. If I don't the song, and all subsequent songs, will play at low volume levels.

- Only playing with the fast forwarding function during a song restores the volume levels from low to high.

- After restoring volume levels to higher levels using the fast forwarding method, the song, and all subsequent songs, play at restored higher volume levels.

- The volume levels do not change when automatically transitioning from one song to another. (if it was low, then it stays low until fiddling with the fast forward function. If it was high, it stays high.)

- The volume levels do not change when forwarding to the next song in the play list. (if it was low, then it stays low until fiddling with the fast forward function. If it was high, it stays high.)

- The Sound Check button does not affect the behaviour (I have problems with volume rectification function as well).

- EQ and Volume Limit functions also do not affect the behaviour.

Notes:

- My iPhone is an iPhone 6 with 16GB, model MG492CH/A, ICC ID starting with 8960315

- I'm running iOS 9.0 (13A244)

- I got a 16GB version (it's a work phone), and the storage is nearly full, with 1.2GB remaining (it alarms me when 1GB is left, and I keep cleaning up stuff as a result).