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.