So I finally got a pair of these. I own (and love) the P7s so was curious to see what the next model up had to offer. I'd read a lot of reviews and this thread and was aware that it was a very divisive headphone.
So far, I find them certainly to have a very unusual tuning and approach to presenting music.
There is a very nice sense of space, probably due to the angled drivers. I remember trying out some headphones from a Japanese brand Crossfeed at a Canjam who have a similar approach with angled cups (and multiple drivers within the cup too), and these sound similar (in terms of space and soundstage) to what I remember hearing there. Seems like angled drivers are a winner for a more out of your head sound.
People say they're hugely bassy but for me I find the emphasis seems to be higher, up in the upper bass or lower mids. There's a strange resonance there which can overpower everything depending on what's playing, e.g. an instrument right at that level. But even without anything specifically focused on that frequency, it is still to the fore - snare drums, for example, instead of sounding snappy having a more round mid whack/thump to them. I've been trying different EQ options on an M11Pro and Plenue S, dropping frequencies around 250-500 Hz, but so far haven't been able to settle on anything that completed resolves the issue. The combination with MojoPoly, which has no EQ, isn't great.
In other senses they do have benefits - more detail and in some ways instruments can sound fuller sound compared to the P7s. In fact, the P7s sound quite bright and anaemic after switching from the P9s, which I never thought I'd say. But the P7s are way easier to listen to because of the above resonance issue and present a more coherent frequency response. The P7s also feel punchier, whereas the attack of notes feels a bit more rounded off on the P9s. Mids and some vocals can also sound strangely veiled and distant on the P9s too.
Ironically (for something designed to play music, i.e. instruments) I find they handle tracks or genres with a more stripped down palette or set of instruments best, probably because it helps avoid problems with that strange resonance spot. I was listening to Myrkur's Mausoleum yesterday on them (female vocals, small choir and sparse guitar/piano), which sounded great - maybe also because the vocals and instruments tended to be higher in frequency than that resonance spot.
So in all a very unusual headphone. Probably the strangest headphone tuning I've heard since the Audioquest Nighthawk/Owl offerings. Maybe I'll keep experimenting before making a decision about whether to keep them. Kind of seemed to me like someone at B&W was onto something interesting, but a prototype during the experimentation stage got released with unresolved issues, as if they needed to keep going with other damping or venting to get a more linear frequency response but management just pushed it out into production before the technicians had finished

I'd be curious to know what EQ adjustments others have found useful.
For reference, the other headphones I own/ed have generally been lower to mid-fi offerings, but include Shure SRH1540, Sennheiser Momentum 2, 630VB, HD6XX, HD58X, Meze 99 classics.