The differences weren't subtle -- the old C5 still sounds sweet, but the C5 Series 2's clarity gains are substantial. Not just in the midrange or treble, the C5 Series 2's deeper and clearly articulated bass lets you more easily follow what's going on down there. (We recently heard similar improvements in the P5 Series 2 on-ear headphones).
The C5 Series 2 is a real treat with great live recordings, like Van Morrison's "Too Late To Stop Now." The sense of being there at the concert is more apparent over the C5 Series 2 than the original C5.
We brought the Hifiman RE-400, an affordable in-ear headphone with an audiophile sound signature, into the mix. With the C5 Series 2, the bells opening Brian Eno's tune "Quartz" have just the right amount of sparkle and shimmer. Over the Hifiman RE-400 the sound thins out and takes on a rougher edge.
With Daft Punk's "Motherboard" the growling bassline that went unnoticed over the RE-400 was given its full due over the C5 Series 2, and the stereo imaging opened up, so the sound feels less stuck inside your head (in general, the C5 Series 2 has a nice, open quality).
The only real downside to the headphone is that since its key strength is its clarity, it can reveal too much of the harshness of less-than-stellar sounding MP3s. But by the same token, that clarity is a big asset with great-sounding recordings.
While the Secure Loop system may not be a perfect fit for everyone, Bowers & Wilkins has taken an already very good sounding headphone and made it sound even better. It's one of the best sounding sub-$200 earphones you can buy and well worth a long look if you're looking for a high-quality, stylish headphone at a midrange price.