Are you absolutely sure you got a perfect seal with them? Your explanation sounds like what happens most of the time with a bad seal...but these could really just be that bad, who knows.

1) The sound is horrible
2) The bass is simply NON-existent and let no one fool you about that!
3) The highs are so high that my ears actually hurt
4) The volume is so much decreased compared to any other earphones I use right now
You just described exactly what happens when you have a bad seal... Recheck your seal and try again. If you can't get a good fit, try 3rd party tips.

I admit it, you may be right. I'm gonna experiment a little more tomorrow and tell you the results. Funny though, I never had issues with sealing before with my sony ericsson earphones (which sound great anyway).

I have these headphones... I don't really like in-ear headphones (I bought it to leave it in my bag, as a backup, in case I don't have my full-sized ones), so I can't compare to others in-ears. But anyway... the sound is not the best, but it's far from horrible IMO. And there's a lot of bass (I'm listening to some drum & bass tracks with it right now). The highs are a bit harsh, though (but since I rarely use these, they have almost no burn-in... maybe this will improve in time).
Regarding the cushions, I tried the medium but switched to the large ones to get a better sound. With the mediums, the seal is really affected.