Low impedance is part of the problem, but high sensitivity is also the culprit with regard to the hissing. Also hissing will be much more noticeable with any headphone that sits in your ear canal. My ER6i headphones also have hissing with X51v, they are high sensitivity, low impedance, canal-phones. There are a couple of things you can do to eliminate the hiss:
1. One thing you can do to eliminate the hissing is add in-line resistance to the headphones. Radioshack sells a small cheap inline volume control that can be put between your x51v and headphones. It will act to both increase the impedance and decrease the sensitivity, and will greatly reduce or eliminate the hissing. At this very moment I am using this with my x51v and ER6i and it eliminates the hiss completely. It means you must turn the volume up more on the x51v of course, which could drain the battery slightly faster. It will also change the sound (frequency response) of the headphones slightly. Shure also sells it's own inline volume control, but it costs more than the Radioshack one.
2. Buy a headphone amp. I know it can be expensive and reduce the portability somewhat, but it will result in the best sound quality. A headphone amp has a very high input impedance (typically over 10kohms), so it will be an easy load for the pda/mp3 player. You must make sure, of course, that the headphone amp itself does not hiss with sensitive headphones. Check
www.head-fi.org for reviews and advice if you want.