Contrary to the long-held advice, I am placing PX200 > PX100 in term of sound quality.
I have had the white PX200 for about month now and I do enjoy them more than the white PX100.
Driven by the latest Benchmark DAC1 headphone out, PX200 consistently had purer mid range and better-extended high. The midrange is less grainy than PX100, and sounds much more natural and neutral. The imaging is stable and the soundstage is defined. I like to focus on the midrange since most of the music I listen to are happening predominantly in the mid-range.
Much like sealed spkr, sealed headphone requires more power to keep the driver in control (deeper bass). I think Benchmark was adequate for this phone. Comparing Benchmark to ipod headphone out, DAC1 headphone out consistently went few notes lower in the bass.
PX200 is ultimately not about mid/upper bass slam, rather PX200 delivers its virtue via the pure mid-range, well-extended and sweet highs, and an overall balance that's akin to headphones costing much more.
PX200 is NOT ideal for ipod crowd and bassheads; but if most of your music is not in the bass range, and you prefer natural mid-range and well-defined soundstage, PX200 is a better choice.