(When I say DAC in the following, I mean DAC / Amp)
I use a Nexus 5 as my file source for my DAC.
You NEED a DAC.
It's funny, I remember when I got my first ES9023 Encore mDAC. I thought it was the greatest thing since sliced bread, compared to my iPhone 6S Plus stock Cirrus DAC / Amp. Even though the improvements were minor, they were quite noticeable and appreciated, as if it opened my eyes to entry-level hifi.
I then moved onto a ES9010K2M Encore mDSD, once I decided that I didn't want a battery-powered DAC. That was somewhat of a mistake; Battery powered is cleaner sound, generally speaking, and doesn't drain your phone as fast. However, at the time, I didn't want to forget to charge my DAC and have a non-functioning DAC without juice. I decided to go with a source-powered DAC, which was nice. As long as the source device had power (Computer or Phone), the DAC had power. However, it had its shortcomings, much of which was related to battery drain. It drained my Pixel XL pretty readily. However, the sound was an incremental improvement over the mDAC, the extra file support was nice, and not requiring wires when using it with a computer was a nice plus.
I recently moved to a ES9028Q2M LH Labs GO2Pro. This was quite a big jump in cost from the other DACs, but when I did an A/B using my computer speakers as the output device, the changes very very noticeable. However, the benefit per $ ratio plummeted. You're spending a lot of money to improve marginally. But in this case, the different in price is over 5 times.
Moral of the story is that the first, cheapest DAC will give you the most bang for your buck. As you progress through this hobby, you will want to get better, clearer, more involving sound, and you will have to pay more $ per better sound, and that curve sharpens up quite considerably once you get past the $100 range.