These are not bad transcriptions, but that's really all they are. They are not arrangements for a string quartet. I can tell you that a string quartet would be bored as hell playing this stuff - especially a good one. Since you've taken out all of the other junk people are used to hearing - like the drums, you gotta compensate by making the string parts a little bit more interesting.
A fantastic example would be the album "Strung out on Ok Computer". All fantastic arrangements and expressively and musically played. You might also want to check out the Vitamin C quartet (though I don't like song selection or playing,) FuzzBox, and the Kronos Quartet.
Also, I couldn't really tell by your samples, but it sounded like you were using section patches instead of single instrument patches.
Good work so far and keep at it!
