There are two big metrics for this type of headphone: overall sound quality and noise cancellation performance. Let's start with the latter. We played a recording of some airplane cabin noise in our audio room and alternated between the QC25 and QC15. Neither completely shut out the drone but the QC25 did a bit of a better job. That said, I did notice a touch more more pressure on my ears, which may bother some people more than others. For me, the pressure is only really noticeable when you don't have any music playing. If you have sensitive eardrums, it may be a deal breaker -- but that will apply to any active noise-canceling headphones, not just the Bose.
Here's the deal on the sound. The QC25 definitely has more exciting sound than the QC 15. It's brighter and clearer and the bass is tighter, which makes it a pretty zippy pair of headphones with faster, more forward sound than its predecessor.
The QC15 has a "High-Low" switch on the headphone cord that allows you to limit the volume. That's missing from the QC25, so you're essentially locked into the high setting. For some people not having ability to choose the lower setting may be an issue, but it didn't bother me at all because I always listened to the QC15 in the high setting.
With certain tracks you feel the sound approaching the edge of harshness but stopping just short. For instance, listening to The Orwells' "Who Needs You" track, it's right at the border of being a little too bright. The same is true of Spoon's "You Got Yr. Cherry Bomb," which has a tambourine running through it that's right at the edge of being accentuated too much and overtaking the song.
Also, with the increased edginess in the treble, badly recorded tracks and poorer MP3 files will end up sounding worse, whereas the older QC15 was better at smoothing over some of those flaws.
I think most people will like the new sound, but there will be those people who may prefer the more laid-back, "warmer" sound of the QC 15. Personally, I prefer a more exciting, zippier headphone, and props to Bose for breaking out of its "safe" sound profile. Some may say it went a little too far, but the QC25 sounds better with most genres, particularly with rock music and more complicated tracks that have a lot of instruments playing at the same time.
If you're an audiophile, well, you probably shouldn't get noise-canceling headphones, for the fact is that really good passive over-ear headphones in this price range, such as the, are going to sound more natural and refined.
I have yet to come across noise-canceling headphones that do the noise-canceling component as well as a Bose model. Its options were also hard to beat for comfort. Sound quality was good but not great.
Now Bose has upped the audio performance in the QuietComfort 25. While it still may not be stellar, you'll be hard-pressed to find superior sound in another noise-canceling model. Yes, it's fairly pricey, but for those seeking premium noise-canceling headphones for travel or just shutting out everyday ambient noise while listening to music or watching video, these QuietComfort headphones are easy to recommend.
Editors' note: CNET contributor Steve Guttenberg and editor Ty Pendlebury listened to the headphones and offered their opinions on their performance. Their views were incorporated into this review.
Correction (September 23, 2014): An earlier version of this review incorrectly stated that the QuietComfort 25s deliver higher volume levels than the earlier QuietComfort 3s based on a testing error. In fact, they do not.