Audio quality has degraded

I love to listen to music and is one of the main reasons why I bought the 128Gb model of the iPhone 6. I have just recently had to have the phone repaired after it started losing WIFI and GPS signal. The antenna flex cable was changed by a local repair person as it was out of warranty. I was glad that this this fixed my problem.

A few days later I bought a new pair of audiophile earphones and when I plugged those in I was dismayed to find that the audio coming out was not the usual high quality but degraded. It was as if I had gone from listening to 320Kbps files to 64Kbps files. Compared to before everything sounded loud bass bloated, slightly muffled and lacking clarity.

I only keep highest quality MP3 and FLAC files on the iPhone and I'm very familiar with my music. I have few high end earphones that are all over the £100 price and all are playing with same audio degradation.

They earphones work fine with iPad Air. I have also placed phone in Safe Mode and it has made no difference to the degraded audio. I have not done any recent updates etc that might have caused a software issue.

When using the phone to make calls the audio through the speaker and mic seems to be fine. But closer listening to music on the phone speaker has revealed that again the sound is not quite as clear and sharp as it should be. I have compared side by side against friends iPhone 6.

Now I'm left wondering if swapping the antenna Flex cable has caused this or its another problem altogether (earphone alignment ?).

Could a faulty/inferior quality of replacement Antenna Flex cable causes this issue ?

Or is this an additional new non related problem on the headphone/charger pcb ?

Loose connection on another flex cable ? Bad contact ?

Advice would be most appreciated.