Long time lurker who never felt the urge to post on here, but having just bought the new Sennheiser PXC 550 and knowing the Bose QC35 quite well, too, let me just say that I can't really agree with intlsubband's findings.
In my humble opinion, the PXC 550 sounds considerably more 'mature' than the Bose QC35, more pleasant / less harsh treble ("sssh"), more detailed / less 'coloured' mids and a more precise bass, too. It needs to be said that right out of the box, the "Club" sound mode was selected, which causes the bass to be bloated, indeed, particularly with bass heavy music. Once the sound modes were switched off (which is not done via touch control btw, but by pressing the small button on the right ear cup a few times), the 'real' sound of these cans is revealed, which, at least from my experience, is rather 'Sennheiser like'. The bass is very enjoyable if you prefer a precise bass, not necessarily headphones for bassheads (but due to the manual equalizing option via the CapTune app, you can pretty much tune it that way), there is a serious 'thump' in the higher bass frequencies, too ("kick bass"), particularly apparent in electronic music (e.g. Prodigy's Firestarter). Voices sound great, both male and female (Johnny Cash, Diana Krall). The best way to describe the sound of these cans is 'balanced', with a slight tendency to the warm side.They're never harsh, even with crappy loudness infested recordings like Metallica's Death Magnetic (ultimate 'harshness' test).
Having played around with them for a few hours so far, I've just tested wireless mode yet, both with my iPhone 6 Plus and via an aptX supporting USB dongle connected to my desktop. Let me just say that aptX does make a difference. The sound is smoother, most apparent in the higher frequencies (e.g. hi-hats), it sounds absolutely brilliant! Just wish Apple would finally support aptX as well.
As far as the noise cancelation is concerned, I have yet to test them in a truly noisy environment, which should be tomorrow at work. A few quick tests were rather promising, though. The 'passive' NC on these is already pretty decent, the cups isolate rather well.
As far as comfort is concerned, I have to agree with intlsubband on the headband, which tends to be a bit uncomfortable if you wear it tight. But the way I prefer to wear my headphones, you hardly notice the headband, especially because the clamping pressure on the ears is quite a bit higher than on the Bose. Fortunately, however, the ear pads are actually softer than the Bose ones, plus the pressure is rather evenly distributed, so long listening session aren't a problem. The form of the ear cups is a plus, IMHO, especially for the large ears of mine. The ears do get warm after long listening sessions, though, these are sealed headphones after all. All in all, I'd probably say that the Bose edge it as far comfort is concerned, the QC35 are a bit more comfortable than the PXC 550, but the PXC 550 are very good in this regard, too.
The PXC 550 are simply the better headphones IMHO. I don't expect them to best Bose on the noise cancelation, but I do expect some very good ANC that is close enough. They are a bit less comfortable to wear, too, but that's being picky, as they're both comfortable headphones. But the most important job of headphones, accurate music reproduction, is simply better. The well tuned, balanced sound makes long listening sessions enjoyable, never tiring.
I'll test them for a few more days, but I think these are a keeper.
