Bluetooth or not

Bluetooth was a replacement for infra-red controllers. (There was actually a better solution at the IEEE but BT won because of the companies that came up with it) Bluetooth is to USB is to Ethernet. Folks don't run data centers on Bluetooth and USB. 

The Cirrus chip that powers your headphones, at least in an iPhone 6 and 6S, has lower noise and distortion than the digital interface even if you plug "colorful bugs" into it.. Plus, these "bugs" do not bypass the 44.1K limit Apple for one has placed on their devices although now they can unfold Stuart's latest marvel I am told. Sneaky.

However, you can load and download 192k/24 bit files, etc., via wifi to your phone and then play them back bypassing the dumbing down digital circuits using the $30 Apple Camera adapter. You then plug your interface(s) into it for true hi rez sound to your headphones. YMMV.

The same general thing goes for preamp input sorry that was the original question. I'd stick with the headphone jack. Belden cables are $30. Preamps have BT in them now? I do some streaming through an iPAD via the headphone jack of some services and if the recording is good at the highest quality level and fastest Arrris wifi 5 GHz the sound is acceptable for casual listening.

A friend of mine was at Munich and told me some fellow this great sound in his STOA system off his iPhone sans "bugs or Bluetooth". I explained the various ways it was done.

I always preferred wired connections for my iphone. Best sound quality for sure.
But today, bluetooth offers great quality.
I always connect my phone with bluetooth in my car. Ok... I don't have the best car system (honda civic) but... lol