The P9 isn't at all portable, believe me, I tried them twice at the shop that sold me my P7 wireless. Frankly I think the P7 wireless has a very similar sound signature and is actually very close in sound quality, albeit not exactly equivalent. I think the only thing the P9 stands out on was the liquid and very rich midrange, it was gorgeous. But the bass has that pronounced mid-bass hump, same as the P7, but I don't think a purported flagship should have such a hump. For the P7 sure, it is an actual portable, and the mid-bass hump there makes sense as it competes with background noise.
Personally I think B&W made a mistake with the P9 in that it suffers from lack of focus. There is no way on any planet the P9 is a portable. Sure, if somebody wants to use it as a portable, no problem, but it is large and it is heavy for that use and not to sound harsh, I think it would look foolish out and about. The P7 is about as large as I think a true portable should be, but that is of course just personal opinion/preference. If you ask me, B&W should have simply made the P9 a flagship, home headphone, put the 1/4" jack on a thicker cable, made the driver larger and really crafted the signature as a statement. In my thinking, it is too similar to the P7 and just doesn't make much sense as conceived. Seriously, what is the P9? Is it a mobile headphone, 22ohm, 111db sensitivity and the default 3.5mm termination seems to say so, but the size, weight and price suggests otherwise. Can't understand the position in the market it is supposed to occupy, but maybe I'm being too critical. I know I cancelled my P9 order after having heard it twice, plus read enough consistent feedback from about three owners who just felt the mid-bass was overcooked and distracting.
I personally think that B&W allowed the design to be tuned with far too much audiophile music ignoring the fact that many customers who purchase expensive headphones like a very wide diversity including bass heavy EDM, pop, electronica, R&B etc. If they tuned using primaril well mastered jazz, acoustic and classical music that might explain why it doesn't really handle bass heavy material as cleanly as you would expect. Anyway, rant over. You should get a KEF Space One to try. If it was wireless I would be all over that baby for sure, but no Bluetooth is a deal breaker for me where portables are concerned.