Audirect's Whistle sounds great. I mean I love the sound of its name. And it sounds good. From an audio perspective, it is more than worth its 99$ price tag. It gets louder than an iPhone (even approaching high-end DAP territory), keeps THD and IMD to inaudible levels, and does so with amazing unloaded performance. Even loaded it does remarkably, again approaching high-end DAP territory. 

Below you'll find charts comparing it to both an iPhone SE and other Lightning dongle DACs.