Where to set frequencies for optimal playback has always been for me trial and error. But I found the settings that I think sound great using my Amazon Mobile app with 5 Band Equalizer along with my A13 Samsung Smart phone using Bluetooth Earbuds. Equalizer on A13 Smart phone set to Normal (flat).
These are close approximations as I don't have precise numbering on graph.
60 Hz +10 db, 230 Hz +2 db, 910 Hz +9 db, 4 KHz 10.5 db, 14 KHz +3 db
These setting to my ears sound the best for most music I listen too but Hard Rock, Heavy Metal especially sound terrific.
My listening has been with Huawei Free Clip Wireless Ear Buds. Occasionally I switch to JBL Tune 130NC
What equalizer settings work for you?
