[ubs28]

My iPhone X and iPad Pro 12.9 sound the same with Wireless headphones.

But my older iPhone 6S plus sound significantly worse.

How is this possible? Is it maybe due to bad isolation of the electronics which bleeds into the signal perhaps?

 

[Zapp_Fan]

If your BT source sounds worse it's possibly because one device supports APT-X or other higher-bitrate codecs, and one doesn't.

Also, make sure your headphones are connected as Headphones and not as a Headset. The headset mode uses a much lower bitrate lossy compression on the audio and sounds like total garbage. It's meant only for phone calls.

 

[bigshot]

The first thing to determine is if the problem is in the iPhone 6 or if it is just the wireless connection as Zapp Fan says. Different versions of the iPhone night have different headphone impedance requirements if you plug straight in. So if you want to add a bit more control, take the line out from both, put them into a switch box run the switch box into a headphone amp and compare that way. (When I was comparing my iPods, I used two identical headphone amps so I could balance levels. But I think line level on iPhones is probably pretty consistent. You might not need to go to that extent.) That will tell you for sure if the problem has to do with the wireless connection or the playback itself. I've had various iPhones and iPods and I've never heard any difference between any of them.

Odds are, it's not a fault of the phone. It's the fault of the way the wireless headphones are connecting to it.

 

[Roseval]

Bluetooth audio (SBC) has 3 quality levels, low, medium and high.
Low = 201 kbs and high=328
Maybe this explains the difference.
http://soundexpert.org/news/-/blogs/bluetooth-audio-quality-a2dp

Another possibility is a mismatch in profiles e..g. if HSP is used audio is encoded to 64 kbit/s