How old is the iPhone? If it's iPhone 5 or older, an old Airport Express is what you want. It can be set up as its own network connection and it avoids using the DAC in the phone which, by Apple's own admission, isn't its strong point. The new Airport Express will work, as will an AppleTV. I compared a $250 BlueTooth receiver with my AVR that has AirPlay and AirPlay sounded better.
AirPort Express has a jack that works as stereo analog out as well as TosLink- I would use the TosLink and connect it to an optical input for the best results. If you keep the iPhone separate from the receiver because of dust, you can put it in a plastic bag and achieve the same thing without the loss. Also, some iPhone cables plug into the connector on the end and not the headphone jack, giving you more output without needing to turn off the limiter in the phone.
You're trying to drive the signal 100', from a device that doesn't have a lot of output in the first place. If you want to use it the way you have been and avoid a lot of expense, another easy way is to use something like this-
https://reverb.com/item/560561-art-headamp-4-headphone-amp-splitter?_aid=pla&pla=1&gclid=CjwKEAjwotmoBRCc6LWd2ZnkuBYSJACyt2quH9Exhpk0iTV8HtoBkLGU5SCO2rTlVVwLh9zLIRsvQhoCsYXw_wcB
The one reason that would keep me from going this route is that boosting the signal increases noise and using the AirPort Express doesn't.