Best of all, because the P5's are so very comfortable, you tend to forget you're wearing them and find yourself not listening to music through a pair of headphones, but instead listening just to your music. I should add here, too, that the closed-back design gives a reasonable level of noise isolation. These factors, as anyone with a modicum of knowledge will tell you, is exactly what you want from a good pair of headphones.
I've had plenty of time to establish that neither my iPhone nor my iPod touch is a particularly good source device for music. Apple may well provide a lossless codec, but even when using high quality IEMs such as the Grado GR8 I'd be kidding myself if I could say for certain that the difference between ALAC and a well-encoded 320kbps MP3 is always discernible. And yet, somehow, the B&W P5s make my iPod touch sound far better than it has any right to.
The low end is prominent, but without becoming overwhelming, which lends these headphones well to the likes of Beethoven's Symphony No. 9 or Mozart's Requiem. Woodwinds and strings in particular have excellent, reverberating warmth of tone. You might argue that the bass is too strong, but at best that would be finicky verging on just plain wrong.
There's superb mid-range clarity as well, with vocals ringing out particularly clearly and an impressive sense of separation between instruments. Topping this off is a crisp, refined high end and a spatial soundstage that never feels crowded. Furthermore, even at high listening volumes the P5's aren't fatiguing on the ears and there's no trace of distortion.
What the high quality of the P5s mostly did, as do so many other high end IEMs and headphones, is remind you just how awful a source device an iPod touch or iPhone actually is. But make no mistake, if I had £250 to spend on a pair of headphones for my iPhone, it would be these I'd buy.
There's no escaping the old maxim of garbage in garbage out, but the Bowers & Wilkins P5 headphones make an iPod sound better than you have any right to expect, and for that they have to be applauded. However, they can only do so much with a substandard input and I can only hope that a set of proper audiophile grade headphones - or even IEMs - will find their way out of Bowers & Wilkins' labs soon enough.