I started reading this thread on my reach to whether I should get the H9i because it was on sales 15% off. I love B&O the brand and the quality headphones they produced. H6 was my first pair then I got the H8, other Beoplay product such as A1, H5, E8. I got the H8i recently because the blue tooth on my H8 starts failing on my and I finally got the H9i today after reading this thread. I have all the H8, H8i, and H9i with me and I did extensive testing and I would love to share my experience with you guys.
(I did all the comparison without ANC)
H6: I love the quality, it has the clearest sound out of the other headphones.
H8: the bass is heavy and it sounded muddy when comparing it with the H6, the H8 does pinch my head when it was new but my H8 is the most comfortable headphones after having it for more than a year, maybe it moulded to the shape of my head.
H8i: I decided to get the H8 since I started experiencing Bluetooth dropout connection from my H8. I was very disappointed to find out the surface was scratched when I unboxed the H8i and it was a sealed package. Since I picked them up from a moving sales, all purchases were final, I couldn't exchange another one nor I can proof to B&O that the product was scratched before I unbox it. Anyway, my first impression of the H8i was a downgrade from H8, the plastic materials made it feels cheap. The USB-C doesn't go into the H8i smoothly. The H8i feels like a step down from H8 but a step up from H4. The H8i sounds muddier compared to the H8. The base is much lighter and the mid doesn't come out as crisp as the H8. I didn't do an in-depth comparison with my Marshall but I do get the impression that my H8i sound very similar to my Marshall MID ANC headphones which costed me much less than the H8i. Anyhow, after playing with the B&O app, I got the sound quality that I like. The lightweight and 30 hours of battery life in the H8i are the only things I'm considering keeping it. However, I already own a pair of Marshall MID ANC which gives me more than 30 hours battery, I will sell my H8i.
H9i: I didn't like what I've got from the H8i and I decided to get the H9i from the same shop which still has the moving sale. I tried the H9i straight from the box, the H9i was running 5.0.1 firmware and it does have all kinds of problems most of you mentioned earlier in this thread. After updating to 5.0.4 all those weird poops were gone. I love the sound, the bass, mid, and high are all balanced. I couldn't compare the H9i with the H6 which I sold it a while ago. But the thinner ear cushion does feel like the H6.