I think i'll drop in now
Listening to the same music, the cheapies actually seem to
have slightly better "presence" in the mid range.
The reason why you're not hearing much difference is dependant on 3 things:
1) The source sound/device (ie soundcard, player)
2) the music/sound and how its recorded. eg: Good CD/DVD quality sound verses one recorded using a cheap dictaphone etc.
3) The speakers/headphones itself. The sennheiser headphones are top quality and most likely have a much more broader range of frequency it can reproduce than your old one, (eg my HD215 is 12Hz-22KHz) cheaper ones probably only in the 22Hz - 20KHz (average human hearing). Thats probably why you're getting a better mid range (800Hz to 1.2KHz) response, since the cheap one's working with less range. Some pro audio heaphones (your's is probably classified in this area) try to replicate a "true" sound by giving it a more flat EQ response. But note all these can be changed with a bit of a EQ adjustment on your media player
i think you got this bit already....
Impedance/Ohms i guess everyone's covered it.. it doesn't hurt your gear by using one or the other, just that you have to remember to turn the volume or amp down a bit if your using the consumer stuff or you'll blow it, pro stuff can take a good beating (electrical wise to a point, but it doesn't mean you can just mistreat it) before they blow.
BTW.. wouldn't 300Ohms need a lot more juice to run than 64Ohms?
Creative X-Fi series are pretty good consumer/amature recording level gear, i'd reckon 100x if not more better than onboard sound. If you can't afford the X-Fi, then the Audigy series are better than nothing, and since they're the older series to creative you may find some on the bargain bins at the shops maybe.. although older they're still pretty good. You can get the internal modules or external module sets with both Audigy and X-Fi series, depends what you want and how much $$ you got..
As for why the difference - doesn't higher impedence result in greater resistance to attenuation from cable length?
i think this has more to do with how the speaker's made (the coil & magnet that drives the speakers back and forth) and the energy it needs to create that sound rather than the cable (even if length of cable does add some resistance to signal going through). I'm not 100% sure and i can't remember (memory blank :|).
If you want to go pro audio cards, there are some.. but i think its gonna be pure overkill for you