iPad mini 5th gen: audio sound is terribly bad

I recently bought a new iPad mini 5th gen (256GB).

I like to listen to music (through youTube or streaming), but the audio sound quality of the iPad mini is terrible. Either DAC or amp is really bad. (I suspect DAC more.)

I tested it using

• iPad mini 5th gen / iPad 6th gen / iPhone 6 plus / Lenovo laptop (average performance)
• AKG K702 / Audio-technica m40x / home audio system (Marantz)

For all test cases, the sound quality is: iPad 6th gen (best) > iPhone 6 plus ~ Lenovo laptop >> iPad mini (worst)

For iPad mini, the mid range is very weak, the treble is very distorted. (The bass is boomy, but that is common for small devices.)

How can the sound of $550 iPad mini be SO WORSE than that of $350 iPad ??? I am very disappointed.

• Does anyone know the DAC or amp difference between iPad mini and iPad?
• Is there any way to improve it on its own?

Thanks.