Ditto to the above. You are wasting your time trying different speaker cables. Try moving the speakers, especially toeing them in. I actually have mine crossfiring about two feet in front of me. Adding room treatment can help too. You said nothing of what your room is like and that is the biggest culprit. Speaker position tweaking costs nothing so is certainly worth a try. Treatment will help more than just the highs.
To much high
This weekend I traded my B&W CDM1's for a pair of beautifully finished home-made speakers, comprising a Morel tweeter, Seas (aluminium) mid/low and Seas woofer in a tall housing. I listened to them on a tube amp in a heavily furnished room, and they sounded awesome. Back home (you guessed it....) I connected them to my transistor amp, in a rather hard-sounding room. The low and mid is everything I ever dreamed of, but the top-end is driving me nuts. Since I don't have the cash to change my amp I'm thinking of changing my (silver) speaker cable, or, as some sort of last resort, placing a resistor before the tweeter. Any (other) suggestions?