Just got a pair of 381F and with the black large earplugs I found a more than expected sound quality. Initially I tried them with small and medium earplugs, but bass was not good enough...also all frequencies were unbalanced.
Music is detailed, with decent soundstage, bass sounds much better then I thought for such cheap IEMs.
Sine tones from audiocheck website sound perfectly flat from around 30Hz up to almost 4kHz, then it's a bump of few dB between 5-6kHz, then a dip around 8kHz, but then everything gets flat again.
These are not very sensitive cans, but they're quite easy to drive. However, I would not pair them with high-impedance headamps (eg.: 10-ohms amplifiers), because I found that bass is lacking when pairing them this way; after all, their impedance is 16-ohms.
I honestly love these cans, despite their microphonic cable, I really think it's the best bang for the buck!
Fits well in my ears.