The vehicle is a Kia Rio UB III 2016, European Edition, 3 Doors.
Worth mentioning, once door panels&co. are taken out, that the inside-design looks very strange, such that there is like a tunnel space on both left and right sides, when looking from the trunk, between the trunk and the space where the back-door "is", you could put your hand from the trunk and come to the side all the way to where the back door is.
- would not be a box issue, the same subwoofer in the same box sounds good in another car/in another enclosed space
- a Home-Subwoofer instead, by keeping it connected to its home amplifier sounds similar
- inverting the phase + time alignment + position settings + etc: slight differences, but with the same overwhelming feeling
If when changing the listening position (like when I lay flat), the bass evens out and seems normal, wouldn't that suggest an acoustic issue?