Alright, I decided to also pick up the Bose QC45 today at a local store, as I just wanted to be able to compare.
These are by far the most comfortable headphones out of all the ones tested. Like, head and shoulders above the rest. Ear cups are 62 x 41 mm. Drivers are angled and at the deepest part are 27mm deep, shallowest part 16mm deep - this is before any earpad compression, so actual depth probably a bit lower. My 67mm long ears have plenty of room inside these deep earcups.
PX7 S2 are 18mm deep at shallowest part, 28mm at deepest part (so, even slightly deeper than the QC45). But at 56 x 37mm size ear cups, definitely on the smaller side.
So the trick for headphone comfort without making really huge earcups is to have angled drivers, where the deepest part is at least 25mm deep. The back of your ear can then go under the earpad (provided the manufacturer ensures that part has no sharp edges or rough material - but that's not the case on the QC45 or PX7 S2).
ANC on the QC45 is excellent, on par with the XM4. Sound profile is neutral with emphasized highs. Main downside is no higher quality codecs than AAC. Baffling why Bose didn't decide to at least include AptX as well. And if that would've meant they needed to bump the price by $10, that would've been completely fine. They're pretty ugly headphones to look at, very utilitarian design (the Bose NC700 look much sleeker, but the QC45 should be more comfortable than those).
It's going to be really tempting to just keep the QC45 and call it a day, as I'm primarily looking for comfort and good ANC in these kinds of headphones. And well, at least it comes with a 2.5mm > 3.5mm cable - and my phone still has a headphone jack, so I can still get good quality audio that way. But these headphones are light and just very comfortable.
I love the sound on the PX7 S2 - but I don't think I could wear them for 8 hours straight on a long flight. I could wear them for 2 hours, maybe 3. QC45 I can already tell I could wear them for that long - and the ANC is superb. But the PX7 S2 at the same time has a certain coziness that almost feels like coming home when putting them on again (and adjusting for just the right fit). I think it partly has to do with the ear cup pads being wide, and distributing the pressure over a larger area, and then of course the very deep cups where my ears don't touch the drivers or any fabric covering it. Kudos to B&W for either way getting the most out of smaller ear cups. Would've been a home run with ear cups at least 5mm longer/wider. (I so hope the PX8 will have bigger ear cups).
I'll make up my mind this week - but for sure the 5909, EAH-A800 and Momentum 4 are all going back. 5909, while sounding absolutely fantastic, just not worth the cost when it has poor ANC - comfort is decent, but could've been better had they used angled drivers, as the ear cups are otherwise pretty big. EAH-A800 has great ANC, but the implementation combined with music is poor, really badly affecting SQ. Momentum 4 has great ANC and SQ, but poor comfort.