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.
NL - no load
SM2 - Earsonics SM2
ES7 - Audio Technica ES7
DT880 - Beyerdynamic DT880/600
24-bit VOL (Full) @+6dB - all targets
24-bit VOL (Full) @+6dB - VS iPhone SE