I think it's as you said implementation, on previous firmware battery drain was higher and now it's better as well as the sound to my ears, maybe they can do more with the chipset in the mini because it could have better power efficiency etc the big brother sounds good now tho
I listened to a cheap AK4490 Dap recently. It was nothing special at all. I say if you like what you're hearing then it's showing that the implementation is far more important than the chip. As Chord would put it, the chip is the canvas; the implementation is the picture