Bluetooth is sonically challenged. The best simple way forward is either an Apple Airport Express or a Chromecast Audio, but both need wifi to connect so will not work on the go. Both have optical digital outputs for unimpaired sound quality. The advantage of the Chromecast is that the actual streaming is not from your phone/tablet, but from the Chromecast itself. Therefore, the phone’s battery is not drained - you can even turn off your phone. For both units there are apps for a great many streaming services - you are not limited to a handful as with many dedicated audio brand streamers.
You could try an Audiengine B1 Bluetooth muscic streamer. They have a 30day return policy if you don't like it. I have one and I stream internet radio from my Samsung Tablet and it works and sounds very good. Pretty surprising actually.
I just went through this with my iPhone 6s. Don’t know if all these options apply to iPhone 7. You have at least four options, two wired and two wireless. The wireless options were already mentioned; Chromecast Audio (optical interface through a DAC or analog to the preamp), Bluetooth if your integrated supports it.
For wired connection, you can use iPhone analog/headphone output and go to preamp or use an apple camera adapter cable and go digital to your DAC USB port. For either of the analog options you need a 3.5mm mini jack to stereo RCA adapter cable.
I tried all of these options except Bluetooth in my set up just over the past weekend and to my ears the iPhone wired through USB was top choice followed by Chromecast through optical connection and Chromecast analog was last. Not huge difference but audible. If you want to go wireless, Chromecast optical connection is a no brainer and can/will sound excellent depending on the quality of your DAC.
The Schiit Modi Multibit. Make sure you get the camera adapter cable which has a USB type A female as well as a Lighting port on the output side. I initially bought a plain Lightning to USB type A female adapter and it did not work properly. I kept getting an error message saying something like not supported on this device after a few minutes of playing. You can either get the Apple OEM or a knock off cable for half the price. The knock off is very well made and is working fine for me. I also have a DACmagic Plus in a different system and it works fine there too. Keep in mind you still need a USB type A to type B, or type A, cable depending on your DAC configuration.