Pros: The best sound quality for a DAP
Accurate but engaging tonality through all frequencies
Large soundstage with precise location for all instruments
Decent power output, enough for all IEMs
Many sound settings and fast-updating firmware provides flexibility in sound style
A variety of connection options
Powered by battery
Low prices compare with other flagship DAPs
Cons: Size and weight
System may be non-intuitive, best sounding firmware only supports WAV format
Lack of EQ options
No LO ports
Insufficient power output compared to a proper desktop AMP
Power supply needed to unleash full power
Conclusion first: the QA390LE is the best DAP I've ever heard, and I think every TOTL IEM lover should have one.
Compare with my already amazing LP6 Platinum, the QA390 maintains a somewhat similar tuning but better in every aspect. The soundstage is larger and more open, the lows are deeper and the highs are detailed but not aggressive. The vocals are forwarded but accurate, making it a very good allrounder for all music genres.
I personally paired it with the Oriolus Traillii and the EA Centurion, which is the combination I love the most so far, but also tried it with different IEMs including the Kinera Loki and VE Phoenix. In every combination the performace of the IEM is significantly elevated compared with the LP6. specifically for the Traillii, with the Centurion and the QA390LE, the already great horizontal soundstage becomes even larger, arguably the largest I've heard in all IEMs, and the previously insufficient depth are significantly improved. It is still not amazing, especially compared with something like the VE Phoenix and the Aura, but it is now close to the performance seen in other common TOTL IEMs like the Aroma Jewel. Overall, the QA390LE could significantly increase the performance of IEMs.