Yeah, it's quite sad I'll have to stop the development for the J3, but the amount of J3 users is getting lower and my J3 is dying, sadly (the battery life is decreasing slowly but constantly). The good side is that I'll be able to use all my programming skills when I'll make stuff for Android.
"Would a skin like the one you're planning to make be available for any Android device or have you not thought that much about it?"
I don't know how Android apps work, but I will try to make it compatible with any Android device.
With very low impedance phones there is some bass roll-off seen in the frequency response graph, but it's not really audible, even with 16 Ohm phones. Human ears aren't good at hearing +/- 2dB differences at 50 Hz and below. With 32 Ohm phones or higher there is nothing to worry about at all.